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RC5052
High Performance Programmable Synchronous DC-DC Controller for Low Voltage Microprocessors
Features
* Optimized for 12V main power * Programmable output from 1.3V to 3.5V using an integrated 5-bit DAC * Remote sense * Active Droop * 85% efficiency typical at full load * Integrated Power Good and Enable/Soft Start functions * Drives N-channel MOSFETs * Overcurrent protection using MOSFET sensing * 20 pin SOIC package * Meets Intel Pentium II & III specifications using minimum number of external components * Adjustable deadtime, frequency * Crowbar protection for overvoltage
Description
The RC5052 is a synchronous mode DC-DC controller IC, optimized for 12V main power, which provides a highly accurate, programmable output voltage for all Pentium II & III CPU applications and other high-performance processors. The RC5052 features remote voltage sensing, adjustable current limit, and active droop for optimal converter transient response. The RC5052 uses a 5-bit D/A converter to program the output voltage from 1.3V to 3.5V. The RC5052 uses a high level of integration to deliver load currents in excess of 16A from a 12V source with minimal external circuitry. Synchronous-mode operation offers optimum efficiency over the entire specified output voltage range. An on-board precision low TC reference achieves tight tolerance voltage regulation without expensive external components, while active droop permits exact tailoring of voltage for the most demanding load transients. The RC5052 also offers integrated functions including Power Good, Output Enable/Soft Start, current limiting, adjustable frequency, adjustable deadtime and overvoltage crowbar protection, and is available in a 20 pin SOIC package.
Applications
* * * * * Power supply for Pentium(R) II & III VRM for Pentium II & III processor Telecom line cards Routers, switches & hubs Programmable step-down power supply
Block Diagram
+5V DTA 15 Rosc 1 VCCA 6 + 4 RS 13 5 11 VCCP 12 HIDRV +12V
OSC +
Digital Control + +
+12V 10 9 LODRV 8 GNDP Power Good 3 PWRGD
VO
5-Bit DAC
20 19181716
1.24V Reference 14 GNDA 2 ENABLE/SS
VID0 VID2 VID4 VID1 VID3
7 OVP
Pentium is a registered trademark of Intel Corporation
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PRODUCT SPECIFICATION
Pin Assignments
ROSC ENABLE/SS PWRGD IFB VFB VCCA OVP GNDP LODRV VCCP
1 2 3 4 5 6 7 8 9
20 19 18
RC5052
17 16 15 14 13 12 11
VID0 VID1 VID2 VID3 VID4 DTA GNDA SW HIDRV VCCQP
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Pin Definitions
Pin Number 1 Pin Name ROSC Pin Function Description Oscillator Resistor Connection. Connect an external resistor to this pin to set the internal oscillator frequency. Layout of this pin is critical to system performance. See Application Information for details. Output Enable/Softstart. A logic LOW on this pin will disable the output. An internal current source allows for open collector control. This pin also doubles as soft start. Power Good Flag. An open collector output that will be logic LOW if the output voltage is not within 12% of the nominal output voltage setpoint. Current Feedback. Pin 4 is used in conjunction with pin 13, as the input for the current feedback control loop. Layout of these traces is critical to system performance. See Application Information for details. Voltage Feedback. Pin 5 is used as the input for the voltage feedback control loop. See Application Information for details regarding correct layout. Analog VCC. Connect to system 5V supply and decouple with a 0.1F ceramic capacitor. Over Voltage Protection. This pin triggers the gate of an external SCR. Power Ground. Return pin for high currents flowing in pins 10 and 11. Connect to a low impedance ground. Low Side FET Driver. Connect this pin to the gate of an N-channel MOSFET for synchronous operation. The trace from this pin to the MOSFET gate should be <0.5". Power VCC. For low side FET driver. Connect to system 12V supply and decouple with a 10 resistor, 4.7F tantalum and a 0.1F ceramic capacitor. High Side Power VCC. For high side FET driver. Connect to system 12V supply and decouple with a 10 resistor, 4.7F tantalum and a 0.1F ceramic capacitor. High Side FET Driver. Connect this pin to the gate of an N-channel MOSFET. The trace from this pin to the MOSFET gate should be <0.5". High side driver source and low side driver drain switching node. Together with IFB pin allows FET sensing for current. Analog Ground. Return path for low power analog circuitry. This pin should be connected to a low impedance system ground plane to minimize ground loops. Dead Time Adjust. Connect an external resistor to this pin to set the dead time. Voltage Identification Code Inputs. These open collector/TTL compatible inputs will program the output voltage over the ranges specified in Table 2. Pull-up resistors are internal to the controller.
2
ENABLE/SS
3 4
PWRGD IFB
5 6 7 8 9
VFB VCCA OVP GNDP LODRV
10 11 12 13 14 15 16-20
VCCP VCCQP HIDRV SW GNDA DTA VID0-4
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PRODUCT SPECIFICATION
RC5052
Absolute Maximum Ratings
Supply Voltage VCCA to GND Supply Voltages VCCP, VCCQP to GND Supply Voltage (VCCQP, Charge Pump) Voltage Identification Code Inputs, VID0-VID4 Junction Temperature, TJ Storage Temperature Lead Soldering Temperature, 10 seconds Power Dissipation, PD Thermal Resistance Junction-to-case, JC 13.5V 15V 18V VCCA 150C -65 to 150C 300C 750mW 105C/W
Recommended Operating Conditions
Parameter Supply Voltage VCCA Input Logic HIGH Input Logic LOW Ambient Operating Temperature Output Driver Supply, VCCP & VCCQP 0 11.4 12 Conditions Min. 4.5 2.0 0.8 70 13.2 Typ. 5 Max. 5.25 Units V V V C V
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PRODUCT SPECIFICATION
Electrical Specifications (VCCA = 5V, VCCP = VCCQP = 12V, VOUT = 2.0V, and TA = +25C using circuit in
Figure 1, unless otherwise noted.) The * denotes specifications which apply over the full operating temperature range. Parameter Output Voltage Output Current Initial Voltage Setpoint ILOAD = 0.8A, VOUT = 2.400V VOUT = 2.000V VOUT = 1.550V TA = 0 to 70C, VOUT = 2.000V VOUT = 1.550V VCCA = 4.75V to 5.25V, VOUT = 2.000V VOUT at ILOAD = 0.8A to Imax 20MHz BW, ILOAD = Imax VOUT = 2.000V VOUT = 1.550V3 ILOAD = 0.8A to Imax,VOUT = 2.000V VOUT = 1.550V3 ILOAD = Imax, VOUT = 2.0V See Figure 5 for tR and tF RDTA = OPEN. See Figure 3 for tDT ROSC = OPEN * 255 80 0 50 Logic HIGH Logic LOW * * * * Current4 * V = 1.5V 5 37.5 115 120 125 93 88 3.74 7.65 4 8.5 19 40 10 17 * * * * * 1.940 1.480 1.900 1.480 45 85 50 50 300 345 1000 100 120 107 112 4.26 9.35 * * * -44 2.397 2.000 1.550 See Table 1 Conditions * Min. 1.3 18 2.424 2.020 1.565 +8 +6 2 -40 11 2.070 1.590 2.100 1.590 60 -36 2.454 2.040 1.580 Typ. Max. 3.5 Units V A V V V mV mV mV mV mVpk V V A % nsec nsec kHz MHz % nsec %Vout V V mA mA A mA %Vout
Output Temperature Drift Line Regulation Internal Droop3 Output Ripple Total Output Variation, Steady State1 Total Output Variation, Transient2 Short Circuit Detect Current Efficiency Output Driver Rise & Fall Time Output Driver Deadtime Oscillator Frequency Oscillator Range Duty Cycle Dead Time Range PWRGD Threshold VCCA UVLO VCCP UVLO VCCA Supply Current VCCP Supply Soft Start Current OVP Ouput High Current OVP Trigger Threshold
Notes: 1. Steady State Voltage Regulation includes Initial Voltage Setpoint, Droop, Output Ripple and Output Temperature Drift and is measured at the converter's VFB sense point. 2. As measured at the converter's VFB sense point. For motherboard applications, the PCB layout should exhibit no more than 0.5m trace resistance between the converter's output capacitors and the CPU. Remote sensing should be used for optimal performance. 3. Using the VFB pin for remote sensing of the converter's output at the load, the converter will be in compliance with Intel's VRM 8.4 specification of +50, -80mV. If Intel specifications on maximum plane resistance from the converter's output capacitors to the CPU are met, the specification of +40, -70mV at the capacitors will also be met. 4. Includes gate current.
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RC5052
Table 1. Output Voltage Programming Codes
VID4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 VID3 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 VID2 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 VID1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 VID0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 Nominal VOUT 1.30V 1.35V 1.40V 1.45V 1.50V 1.55V 1.60V 1.65V 1.70V 1.75V 1.80V 1.85V 1.90V 1.95V 2.00V 2.05V 2.0V 2.1V 2.2V 2.3V 2.4V 2.5V 2.6V 2.7V 2.8V 2.9V 3.0V 3.1V 3.2V 3.3V 3.4V 3.5V
Note: 1. 0 = processor pin is tied to GND. 1 = processor pin is open.
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RC5052
PRODUCT SPECIFICATION
Typical Operating Characteristics (VCCA = 5V, VCCP = VCCQP = 12V, and TA = +25C using circuit
in Figure 1, unless otherwise noted.)
Efficiency vs. Output Current 2.04 88 86 84 Efficiency (%) 82 80 78 76 74 72 70 68 66 64 VOUT = 1.550V VOUT = 2.000V 2.03 2.02 2.01 VOUT (V) 2.00 1.99 1.98 1.97 1.96 1.95 1.94 0 3
Droop, VOUT = 2.0V
6
9
12
15
18
Output Current (A) 0 3 6 9 12 15 18 Output Current (A)
Output Voltage vs. Output Current 3.5 3.0 2.5 VOUT (V) 2.0 1.5 1.0 0.5 0 0 5 10 15 20 25 Output Current (A)
Output Programming, VID4 = 0 2.1 1.9 VOUT (V) VOUT (V) 1.7 1.5 1.3 1.1 1.30 3.5 3.0 2.5 2.0 1.5 1.0 1.40 1.50 1.60 1.70 1.80 1.90 2.00
Output Programming, VID4 = 1
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 DAC Setpoint
DAC Setpoint
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RC5052
Typical Operating Characteristics (continued)
Output Ripple, 2.0V @ 18A Transient Res p onse, 12.5A to 0.5A
VOUT (20mV/div)
VOUT (50mV/div)
1.590V 1.550V 1.480V
Time (1s/division)
Time (20s/division)
Transient Response, 0.5A to 12.5A
VOUT (50mV/div)
1.590V 1.550V 1.480V
Time (20s/division)
Switching Waveforms, 18A Load
Output Startup, System Power-up
5V/div
HIDRV pin
5V/ div
LODRV pin
Time (1s/division)
VOUT (1V/div)
VIN (2V/div)
Time (10ms/division)
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PRODUCT SPECIFICATION
Typical Operating Characteristics (continued)
Output Startup from Enable 2.042 VOUT (1V/div) ENABLE (2V/div) 2.040 2.038 VOUT (V) 2.036 2.034 2.030 2.028 2.026 0 Time (10ms/division) 25 Temperature (C) 70 100 VOUT Temperature Variation
Application Circuit
+5V F1* +12V CIN* R1 33 C2 1F C5 1F R2 4.7 Q1 R3 4.7 11 12 13 14 15 16 17 18 19 20 10 9 8 7 6 5 4 3 2 1 Q2 L1 (Optional) 2.5H D2 6.2V C3 0.1F R10 10 L2 1.3H D1 MBRD835L
R6 10 Q3 2N6394 C1 1F
R9 3m VO COUT*
R11 200 Optional R8 (Optional) VID4 VID3 VID2 VID1 VID0
U1 RC5052
R5 6.24K VCC R4 10K PWRGD C6 0.1F R7 (Optional) *Refer to Table 3 for values of COUT F1, and CIN.
ENABLE/SS C4 0.1F
Figure 1. 12V Main Power Application Circuit for Coppermine/Camino Processors, Including Crowbar
(Typical Design)
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PRODUCT SPECIFICATION
RC5052
Table 2. RC5052 Application Bill of Materials for Coppermine/Camino Processors, Including Crowbar (Typical Design)
Reference C1-2, C5 C3-4,6 CIN COUT D1 D2 L1 L2 Q1-2 Manufacturer Part # AVX TAJB475M010R5 Panasonic ECU-V1H104ZFX Sanyo 10MV1200GX Sanyo 6MV1500GX Fairchild MBRS320 Fairchild MMSZ5233B Any Any Fairchild FDP6030L or FDB6030L Motorola 2N6394 Any Any Any Any Any Any Any Any Any Littelfuse R251 005 Fairchild RC5052M Quantity 3 3 3 8 1 1 Optional 1 2 Description 1F, 16V Capacitor 100nF, 50V Capacitor 1200F, 10V Electrolytic IRMS = 2A 1500F, 6.3V Electrolytic 8A Schottky Diode 6.2V Zener 2.5H, 10A Inductor 1.3H, 20A Inductor N-Channel MOSFET (TO-220 or TO-263) SCR 33 4.7 10K 6.24K 10 Sets frequency. Sets deadtime. 3.0m 200 5A Fast Fuse DC/DC Controller PCB Trace Resistor Must be used when Q3 present. DCR ~ 6m See Note 1. DCR ~ 2m RDS(ON) = 20m @ VGS = 4.5V See Note 2. ESR 44m Requirements/Comments
Q3 R1 R2-3 R4 R5 R6, R10 R7 R8 R9 R11 F1 U1
Optional 1 2 1 1 2 Optional Optional 1 Optional Optional 1
Notes: 1. Inductor L1 is recommended to isolate the 5V input supply from noise generated by the MOSFET switching, and to comply with Intel dI/dt requirements. L1 may be omitted if desired. 2. For designs using the TO-220 MOSFETs, heatsinks with thermal resistance SA < 20C/W should be used. For designs using the TO-263 MOSFETs, adequate copper area should be used. For details and a spreadsheet on MOSFET selections, refer to Applications Bulletin AB-8.
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RC5052
PRODUCT SPECIFICATION
+12V F1* +5V CIN* R1 33 C5 1F C2 1F R2 4.7 Q1 C1 4.7F R10 200 Optional R8 (Optional) VID4 VID3 VID2 VID1 VID0 11 12 13 14 15 16 17 18 19 20 10 9 8 7 6 5 4 3 2 1 R3 4.7 L2 1.3H D1 MBRD835L R9 3m VO Q2 COUT* L1 (Optional) 2.5H
R6 10 Q3 2N6394
U1 RC5052
C3 0.1F
R5* 6.24K VCC R4 10K PWRGD C6 0.1F R7 (Optional) *Refer to Table 3 for values of COUT, R5, and CIN.
ENABLE/SS C4 0.1F
Figure 2. Typical 5V Main Power Application Circuit, Including Crowbar
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RC5052
Table 3. RC5052 Application Bill of Materials for Coppermine/Camino Processors
(Typical Design) Reference C1 C2, C5 C3-4,6 CIN COUT D1 L1 L2 Q1-2 Manufacturer Part # AVX TAJB475M010R5 Panasonic ECU-V1C105ZFX Panasonic ECU-V1H104ZFX Sanyo 10MV1200GX Sanyo 6MV1500GX Fairchild MBRD835L Any Any Fairchild FDP6030L or FDB6030L Motorola 2N6394 Any Any Any Any Any Any N/A Any Any Littelfuse R251 005 Fairchild RC5052M Quantity 1 2 3 3 8 1 Optional 1 2 Description 4.7F, 10V Capacitor 1F, 16V Capacitor 100nF, 50V Capacitor 1200F, 10V Electrolytic IRMS = 2A 1500F, 6.3V Electrolytic 8A Schottky Diode 2.5H, 10A Inductor 1.3H, 20A Inductor N-Channel MOSFET (TO-220 or TO-263) SCR 33 4.7 10K 6.24K 10 Sets frequency. Sets deadtime. 3.0m 200 5A Fast Fuse DC/DC Controller PCB Trace Resistor DCR ~ 6m See Note 1. DCR ~ 2m RDS(ON) = 20m @ VGS = 4.5V See Note 2. ESR 44m Requirements/Comments
Q3 R1 R2-3 R4 R5 R6 R7 R8 R9 R10 F1 U1
Optional 1 2 1 1 1 Optional Optional 1 Optional Optional 1
Notes: 1. Inductor L1 is recommended to isolate the 5V input supply from noise generated by the MOSFET switching, and to comply with Intel dI/dt requirements. L1 may be omitted if desired. 2. For designs using the TO-220 MOSFETs, heatsinks with thermal resistance SA < 20C/W should be used. For designs using the TO-263 MOSFETs, adequate copper area should be used. For details and a spreadsheet on MOSFET selections, refer to Applications Bulletin AB-8.
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RC5052
PRODUCT SPECIFICATION
Test Parameters
tR 90% 10% tDT 2V 2V 90% 2V tDT 2V LODRV 10% tF HIDRV
Figure 3. Output Drive Timing Diagram
Application Information
The RC5052 Controller
The RC5052 is a programmable synchronous DC-DC controller IC optimized for 12V main power. It may also be used in a 5V main power circuit when the crowbar and additional programmability of the RC5052 are desired. When designed around the appropriate external components, the RC5052 can be configured to deliver more than 16A of output current, as appropriate for the Katmai and Coppermine and other processors. The RC5052 functions as a fixed frequency PWM step down regulator.
There is an additional comparator in the analog control section whose function is to set the point at which the RC5052 current limit comparator disables the output drive signals to the external power MOSFETs.
High Current Output Drivers
The RC5052 contains two identical high current output drivers that utilize high speed bipolar transistors in a push-pull configuration. The drivers' power and ground are separated from the chip's power and ground for switching noise immunity. The high-side driver's power supply pin, VCCQP, is supplied from an external 12V source through a series resistor. The resulting voltage is sufficient to provide the gate to source drive to the external MOSFETs required in order to achieve a low RDS,ON. The low-side driver's power supply pin, VCCP, is supplied from the same source as VCCQP. The VCCQP pin should be run as a charge pump for +12V Main Power, as shown in Figure 1.
Main Control Loop
Refer to the RC5052 Block Diagram on page 1. The RC5052 implements "summing mode control", which is different from both classical voltage-mode and current-mode control. It provides superior performance to either by allowing a large converter bandwidth over a wide range of output loads. The control loop of the regulator contains two main sections: the analog control block and the digital control block. The analog section consists of signal conditioning amplifiers feeding into a comparator which provides the input to the digital control block. The signal conditioning section accepts input from the IFB (current feedback) and VFB (voltage feedback) pins and sets up two controlling signal paths. The first, the voltage control path, amplifies the difference between the VFB signal and the reference voltage from the DAC and presents the output to one of the summing amplifier inputs. The second, current control path, takes the difference between the IFB and SW pins when the high-side MOSFET is on, reproducing the voltage across the MOSFET and thus the input current; it presents the resulting signal to another input of the summing amplifier. These two signals are then summed together. This output is then presented to a comparator looking at the oscillator ramp, which provides the main PWM control signal to the digital control block. The digital control block takes the analog comparator input and the main clock signal from the oscillator to provide the appropriate pulses to the HIDRV and LODRV output pins. These two outputs control the external power MOSFETs.
Internal Voltage Reference
The reference included in the RC5052 is a precision band-gap voltage reference. Its internal resistors are precisely trimmed to provide a near zero temperature coefficient (TC). Based on the reference is the output from an integrated 5-bit DAC. The DAC monitors the 5 voltage identification pins, VID0-4. When the VID4 pin is at logic HIGH, the DAC scales the reference voltage from 2.0V to 3.5V in 100mV increments. When VID4 is pulled LOW, the DAC scales the reference from 1.30V to 2.05V in 50mV increments. All VID codes are available, including those below 1.80V. The output voltage may be changed while the converter is on by changing the VID codes; however, it is necessary to do so in 1-bit steps, to avoid triggering the overvoltage protection.
Power Good (PWRGD)
The RC5052 Power Good function is designed in accordance with the Pentium II DC-DC converter specifications and provides a continuous voltage monitor on the VFB pin. The circuit compares the VFB signal to the VREF voltage and outputs an active-low interrupt signal to the CPU should the power supply voltage deviate more than 12% of its nominal setpoint. The output is guaranteed open-collector high when the power supply voltage is within 7% of its nominal setpoint.
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RC5052
The Power Good flag provides no other control function to the RC5052.
Design Considerations and Component Selection
Additional information on design and component selection may be found in Fairchild's Application Note 57.
Output Enable/Soft Start (ENABLE/SS)
The RC5052 will accept an open collector/TTL signal for controlling the output voltage. The low state disables the output voltage. When disabled, the PWRGD output is in the low state. Even if an enable is not required in the circuit, this pin should have attached a capacitor (typically 100nF) to softstart the switching. A larger value may occasionally be required if the converter has a very large capacitor at its output.
MOSFET Selection
This application requires N-channel Logic Level Enhancement Mode Field Effect Transistors. Desired characteristics are as follows: * Low Static Drain-Source On-Resistance, RDS,ON < 20m (lower is better) * Low gate drive voltage, VGS = 4.5V rated * Power package with low Thermal Resistance * Drain-Source voltage rating > 15V.
Over-Voltage Protection
The RC5052 constantly monitors the output voltage for protection against over-voltage conditions. If the voltage at the VFB pin exceeds the selected program voltage, an over-voltage condition is assumed and the RC5052 disables the output drive signal to the external high-side MOSFET, and drives the OVP pin high. This is designed to drive the gate of an external SCR, which blows a fuse, disconnecting the short from the power bus. See Figures 1-2 for the suggested implementation.
The on-resistance (RDS,ON) is the primary parameter for MOSFET selection. The on-resistance determines the power dissipation within the MOSFET and therefore significantly affects the efficiency of the DC-DC Converter. For details and a spreadsheet on MOSFET selection, refer to Applications Bulletin AB-8.
Inductor Selection Oscillator
The RC5052 oscillator free runs at 300 MHz, and may be adjusted from 80KHz to 1MHz as desired. Higher frequencies will permit smaller components, while decreasing efficiency. A typical operating frequency is 300KHz. The frequency may be adjusted up with a resistor to ground on pin 1, according to the formula:
40K f = 300kHz * 1+ ROSC
Choosing the value of the inductor is a tradeoff between allowable ripple voltage and required transient response. The system designer can choose any value within the allowed minimum to maximum range in order to either minimize ripple or maximize transient performance. The first order equation (close approximation) for minimum inductance is:
Lmin = (Vin - Vout) f x Vout Vin ESR x Vripple
where: and may be adjusted down with a resistor to 5V on pin 1, according to the formula:
160K f = 300kHz * 1ROSC
Vin = Input Power Supply Vout = Output Voltage f = DC/DC converter switching frequency ESR = Equivalent series resistance of all output capacitors in parallel Vripple = Maximum peak to peak output ripple voltage budget. The first order equation for maximum allowed inductance is:
Lmax = 2C0 (Vin - Vout) Dm Vtb Ipp2
Dead Time
The RC5052 can control the deadtime, that is, the time between when the high-side MOSFET is turned off and the low-side MOSFET is turned on, and vice versa. Longer dead times are appropriate when using multiple MOSFETs in parallel, or when MOSFETs with larger gate capacitance are used. The dead time may be adjusted with a resistor to ground on pin 15, according to the formula:
TDT = 15nsec + 40K * RDTA 10nsec * 10K + K 40K + RDTA
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PRODUCT SPECIFICATION
where: Co = The total output capacitance Ipp = Maximum to minimum load transient current Vtb = The output voltage tolerance budget allocated to load transient Dm = Maximum duty cycle for the DC/DC converter (usually 95%). Some margin should be maintained away from both Lmin and Lmax. Adding margin by increasing L almost always adds expense since all the variables are predetermined by system performance except for Co, which must be increased to increase L. Adding margin by decreasing L can be done by purchasing capacitors with lower ESR. The RC5052 provides significant cost savings for the newer CPU systems that typically run at high supply current.
The converter exhibits a normal load regulation characteristic until the voltage across the MOSFET exceeds the internal short circuit threshold of 50A * 8.2K = 410mV, which occurs at 410mV/25m = 16.4A. (Note that this current limit level can be as high as 410mV/15m = 27A, if the MOSFET has typical RDS,on rather than maximum, and is at 25C. This is the reason for using the external sense resistor.) At this point, the internal comparator trips and signals the controller to reduce the converter's duty cycle to approximately 20%. This causes a drastic reduction in the output voltage as the load regulation collapses into the short circuit control mode. With a 40m output short, the voltage is reduced to 16.4A * 40m = 650mV. The output voltage does not return to its nominal value until the output current is reduced to a value within the safe operating range for the DC-DC converter.
RC5052 Short Circuit Current Characteristics
The RC5052 short circuit current characteristic includes a hysteresis function that prevents the DC-DC converter from oscillating in the event of a short circuit. The short circuit limit is set with the R5 resistor, as given by the formula
R5 = ISC RDS, on IDetect
RS IFB RSENSE SW VOUT
Figure 5. Precision Current Sensing
with IDetect 50A, ISC the desired current limit, and RDS,on the high-side MOSFET's on resistance. Remember to make the R5 large enough to include the effects of initial tolerance and temperature variation on the MOSFET's RDS,on. However, the value of R5 must be < 8.3K. Alternately, use of a sense resistor in series with the source of the MOSFET, as shown in Figure 5, eliminates this source of inaccuracy in the current limit. Note the addition of one diode, which is necessary for proper operation of this circuit. As an example, Figure 4 shows the typical characteristic of the DC-DC converter circuit with an FDB6030L high-side MOSFET (RDS = 20m maximum at 25C * 1.25 at 75C = 25m) and a 8.2K RS.
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 5 10 15 20 25 Output Current (A)
Schottky Diode Selection
The application circuit of Figure 1 shows a Schottky diode, D1, which is used as a free-wheeling diode to assure that the body-diode in Q2 does not conduct when the upper MOSFET is turning off and the lower MOSFET is turning on. It is undesirable for this diode to conduct because its high forward voltage drop and long reverse recovery time degrades efficiency, and so the Schottky provides a shunt path for the current. Since this time duration is very short, the selection criterion for the diode is that the forward voltage of the Schottky at the output current should be less than the forward voltage of the MOSFET's body diode.
Output Filter Capacitors
The output bulk capacitors of a converter help determine its output ripple voltage and its transient response. It has already been seen in the section on selecting an inductor that the ESR helps set the minimum inductance, and the capacitance value helps set the maximum inductance. For most converters, however, the number of capacitors required is determined by the transient response and the output ripple voltage, and these are determined by the ESR and not the capacitance value. That is, in order to achieve the necessary ESR to meet the transient and ripple requirements, the capacitance value required is already very large.
Figure 4. RC5052 Short Circuit Characteristic
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RC5052
The most commonly used choice for output bulk capacitors is aluminum electrolytics, because of their low cost and low ESR. The only type of aluminum capacitor used should be those that have an ESR rated at 100kHz. Consult Application Bulletin AB-14 for detailed information on output capacitor selection. The output capacitance should also include a number of small value ceramic capacitors placed as close as possible to the processor; 0.1F and 0.01F are recommended values.
Additional droop can be added to the active droop using a discrete resistor (typically a PCB trace) outside the control loop, as shown in Figure 2. This is typically only required for the most demanding applications, such as for the next generation Intel processor (tolerance = +40/-70mV).
PCB Layout Guidelines
* Placement of the MOSFETs relative to the RC5052 is critical. Place the MOSFETs such that the trace length of the HIDRV and LODRV pins of the RC5052 to the FET gates is minimized. A long lead length on these pins will cause high amounts of ringing due to the inductance of the trace and the gate capacitance of the FET. This noise radiates throughout the board, and, because it is switching at such a high voltage and frequency, it is very difficult to suppress. * In general, all of the noisy switching lines should be kept away from the quiet analog section of the RC5052. That is, traces that connect to pins 9, 10, 11, 12 and 13 (LODRV, VCCP, VCCQP, HIDRV and SW) should be kept far away from the traces that connect to pins 4 through 6, and pin 14. * Place the 0.1F decoupling capacitors as close to the RC5052 pins as possible. Extra lead length on these reduces their ability to suppress noise. * Each VCC and GND pin should have its own via to the appropriate plane. This helps provide isolation between pins. * Place the MOSFETs, inductor, and Schottky as close together as possible for the same reasons as in the first bullet above. Place the input bulk capacitors as close to the drains of the high side MOSFETs as possible. In addition, placement of a 0.1F decoupling cap right on the drain of each high side MOSFET helps to suppress some of the high frequency switching noise on the input of the DC-DC converter. * Place the output bulk capacitors as close to the CPU as possible to optimize their ability to supply instantaneous current to the load in the event of a current transient. Additional space between the output capacitors and the CPU will allow the parasitic resistance of the board traces to degrade the DC-DC converter's performance under severe load transient conditions, causing higher voltage deviation. For more detailed information regarding capacitor placement, refer to Application Bulletin AB-5. * A PC Board Layout Checklist is available from Fairchild Applications. Ask for Application Bulletin AB-11.
Input Filter
The DC-DC converter design may include an input inductor between the system +5V supply and the converter input as shown in Figure 6. This inductor serves to isolate the +5V supply from the noise in the switching portion of the DC-DC converter, and to limit the inrush current into the input capacitors during power up. A value of 2.5H is recommended. It is necessary to have some low ESR aluminum electrolytic capacitors at the input to the converter. These capacitors deliver current when the high side MOSFET switches on. Figure 6 shows 3 x 1000F, but the exact number required will vary with the speed and type of the processor. For the top speed Katmai and Coppermine, the capacitors should be rated to take 9A and 6A RMS of ripple current respectively. Capacitor ripple current rating is a function of temperature, and so the manufacturer should be contacted to find out the ripple current rating at the expected operational temperature. For details on the design of an input filter, refer to Applications Bulletin AB-15.
2.5H 5V 0.1F Vin 1000F, 10V Electrolytic
Figure 6. Input Filter
Active Droop
The RC5052 includes active droop: as the output current increases, the output voltage drops. This is done in order to allow maximum headroom for transient response of the converter. The current is sensed by measuring the voltage across the high-side MOSFET during its on time. Note that this makes the droop dependent on the temperature of the MOSFET. However, when the formula given for selecting RS (current limit) is used, there is a maximum droop possible (-40mV), and when this value is reached, additional drop across the MOSFET will not cause any increase in droop--until current limit is reached.
Additional Information
For additional information contact Fairchild Semiconductor at http://www.fairchildsemi.com/cf/tsgn.htm or contact an authorized representative in your area.
REV. 1.3.2 8/27/01
15
RC5052
PRODUCT SPECIFICATION
Appendix
Worst-Case Formulae for the Calculation of Cout, R5, and Cin (Circuit of Figure 1 Only)
The following formulae design the RC5052 for worst-case operation, including initial tolerance and temperature dependence of all of the IC parameters (initial setpoint, reference tolerance and tempco, active droop tolerance, current sensor gain), the initial tolerance and temperature dependence of the MOSFET, and the ESR of the capacitors. The following information must be provided: VT+, the value of the positive transient voltage limit; |VT-|, the absolute value of the negative transient voltage limit; IO, the maximum output current; Vnom, the nominal output voltage; Vin, the input voltage (typically 5V); ESR, the ESR of the output caps, per cap (44m for the Sanyo parts shown in this datasheet);
The value of R5 must be 8.3K. If a greater value is calculated, RD must be reduced. Number of capacitors needed for Cout = the greater of:
X= ESR * IO VT-
or
ESR * IO VT+ -0.004 * Vnom + 14400 * IO * RD 18 * R5 * 1.1
Y=
Example: Suppose that the transient limits are 134mV, current I is 14.2A, and the nominal voltage is 2.000V, using MOSFET current sensing and the usual caps. We have VT+ = |VT-| = 0.134, IO = 14.2, Vnom = 2.000, and RD = 0.67. We calculate:
2
2.000 14.2 * 5 Cin = 2
-
2.000 5
RD, the on-resistance of the MOSFET (10m for the FDB7030); RD, the tolerance of the current sensor (usually about 67% for MOSFET sensing, including temperature). Irms, the rms current rating of the input caps (2A for the Sanyo parts shown in this datasheet).
= 3.47 4 caps
R5 =
14.2 * 0.010 * (1 + 0.67) * 1.10 50 * 10-6
= 5.2K
X= 2 IO * Cin = Irms IO* RD * (1 + RD) * 1.10 50 * 10-6 Vnom Vin - Vnom Vin Y=
0.044 * 14.2 0.134 0.044 * 14.2
= 4.66
= 4.28 18 * 10400 * 1.1
0.134 - 0.004 * 2.000 +
14400 * 14.2 * 0.020
R5 =
Since X > Y, we choose X, and round up to find we need 5 capacitors for COUT. A detailed explanation of this calculation may be found Applications Bulletin AB-XX.
16
REV. 1.3.2 8/27/01
PRODUCT SPECIFICATION
RC5052
Mechanical Dimensions
20 Lead SOIC
Notes: Notes 1. Dimensioning and tolerancing per ANSI Y14.5M-1982. 2. "D" and "E" do not include mold flash. Mold flash or protrusions shall not exceed .010 inch (0.25mm). 3. "L" is the length of terminal for soldering to a substrate. 4. Terminal numbers are shown for reference only. 5 2 2 5. "C" dimension does not include solder finish thickness. 6. Symbol "N" is the maximum number of terminals.
Symbol A A1 B C D E e H h L N ccc
Inches Min. Max.
Millimeters Min. Max.
.093 .104 .004 .012 .013 .020 .009 .013 .496 .512 .291 .299 .050 BSC .394 .010 .016 20 0 -- 8 .004 .419 .029 .050
2.35 2.65 0.10 0.30 0.33 0.51 0.23 0.32 12.60 13.00 7.40 7.60 1.27 BSC 10.00 0.25 0.40 20 0 -- 8 0.10 10.65 0.75 1.27
3 6
20
11
E
H
1
10
D A e B A1 SEATING PLANE -C- LEAD COPLANARITY ccc C
h x 45 C
L
REV. 1.3.2 8/27/01
17
RC5052
PRODUCT SPECIFICATION
Ordering Information
Product Number RC5052M Package 20 pin SOIC
DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
www.fairchildsemi.com
8/27/01 0.0m 011 Stock#DS30005052 2001 Fairchild Semiconductor Corporation

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