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Tem ommercial Extended C 0C to 70C -2 quipment Handheld E for Portable
G FEATURIN rature Range pe
July 2000
ML4863*
High Efficiency Flyback Controller
GENERAL DESCRIPTION
The ML4863 is a flyback controller designed for use in multi-cell battery powered systems such as PDAs and notebook computers. The flyback topology is ideal for systems where the battery voltage can be either above or below the output voltage, and where multiple output voltages are required. The ML4863 uses the output voltage as the feedback control signal to the current mode variable frequency flyback controller. In addition, a synchronous rectifier control output is supplied to provide the highest possible conversion efficiency (greater than 85% efficiency over a 1mA to 1A load range). The ML4863 has been designed to operate with a minimum number of external components to optimize space and cost.
FEATURES
s
Variable frequency current mode control and synchronous rectification for high efficiency Minimum external components Guaranteed start-up and operation over a wide input voltage range (3.15V to 15V) High frequency operation (>200kHz) minimizes the size of the magnetics Flyback topology allows multiple outputs in addition to the regulated 5V Built-in overvoltage and current limit protection
s s
s
s
s
*Some Packages Are Obsolete
BLOCK DIAGRAM
SHDN 3 VIN VFB 4.5V LDO BIAS & UVLO VFB
VCC VCC
5
1 4
- +
VFB VREF VCC GND 8 OUT 1 A1 6
I CURRENT COMPARATOR
+ -
COMP
SWITCHING CONTROL
18mV Rgm 18mV
- +
COMP
RECTIFIER COMPARATOR BLANKING
VCC
CROSS-CONDUCTION PROTECTION OUT 2
A2
7
SENSE 2
1
ML4863
PIN CONFIGURATION
ML4863 8-Pin SOIC (S08)
VIN SENSE SHDN VFB 1 2 3 4 8 7 6 5 GND OUT 2 OUT 1 VCC
TOP VIEW
PIN DESCRIPTION
PIN NAME FUNCTION PIN NAME FUNCTION
1 2 3
VIN SENSE SHDN
Battery input voltage Secondary side current sense
5 6
VCC OUT 1 OUT 2 GND
Internal power supply node for connection of a bypass capacitor Flyback primary switch MOSFET driver output Flyback synchronous rectifier MOSFET driver output Analog signal ground
Pulling this pin high initiates a shutdown mode to minimize battery drain Feedback input from transformer secondary, and supply voltage when VOUT > 4.5V
7 8
4
VFB
2
ML4863
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VIN ................................................................. GND - 0.3V to 18V Voltage on any other pin ........................... GND - 0.3V to 7V Source or Sink Current (OUT1 & OUT2) ...................... 1A Junction Temperature .............................................. 150C Storage Temperature Range...................... -65C to 150C Lead Temperature (Soldering 10 Sec.) ..................... 260C Thermal Resistance (qJA) .................................... 160C/W
OPERATING CONDITIONS
Temperature Range ML4863CS ................................................. 0C to 70C ML4863ES ............................................. -20C to 70C ML4863IS .............................................. -40C to 85C VIN Operating Range ................................... 3.15V to 15V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VIN = 12V, TA = Operating Temperature Range (Note 1)
SYMBOL OSCILLATOR tON ON Time C Suffix E/I Suffix Minimum Off Time VFB REGULATION Total Variation OUTPUT DRIVERS OUT1 Rise Time OUT1 Fall Time OUT2 Rise Time OUT2 Fall Time CLOAD = 3nF, 20% to 90% of VCC CLOAD = 3nF, 90% to 20% of VCC CLOAD = 3nF, 20% to 90% of VCC Continuous Mode, CLOAD = 3nF, 90% to 20% of VCC Discontinuous Mode, CLOAD = 3nF, 90% to 20% of VCC SHDN Input High Voltage Input Low Voltage Input Bias Current SENSE SENSE Threshold - Full Load SENSE Threshold - Short Circuit CIRCUIT PROTECTION Undervoltage Lockout Start-up Threshold Undervoltage Lockout Hysteresis 3.0 0.5 3.15 0.6 V V VIN = 5V, VFB = VFB (No Load) - 100mV VFB = 0V 130 150 160 235 mV mV SHDN = 5V 5 2.0 0.8 10 V V A 60 60 60 60 125 70 70 70 70 150 ns ns ns ns ns Line, Load, & Temp 4.85 5 5.15 V VFB = 0V 2.1 2.1 450 2.5 2.5 650 2.8 2.95 850 s s ns PARAMETER CONDITIONS MIN TYP MAX UNITS
3
ML4863
ELECTRICAL CHARACTERISTICS
SYMBOL SUPPLY IFB IIN VFB Quiescent Current VIN Shutdown Current SHDN = 5V SHDN = 5V, VIN < 6V VCC VCC Output Voltage VFB = 0V, VIN = 15V, CVCC = 0.1F VFB = 0V, VIN = 6V, CVCC = 0.1F VFB = 0V, VIN = 3.15V, CVCC = 0.1F VFB = 5V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
(Continued)
CONDITIONS MIN TYP MAX UNITS
PARAMETER
100 20 5 4.5 4.0 2.8 4.5 5
150 25 10 5.5 5.0
A A A V V V
5.15
V
4
ML4863
FUNCTIONAL DESCRIPTION
The ML4863 utilizes a flyback topology with constant ontime control. The circuit determines the length of the offtime by waiting for the inductor current to drop to a level determined by the feedback voltage (VFB). Consequently, the current programming is somewhat unconventional because the valley of the current ripple is programmed instead of the peak. The controller automatically enters burst mode when the programmed current falls below zero. Constant on-time control therefore features a transition into and out of burst mode which does not require additional control circuitry. The control circuit is made up of four distinctive blocks; the constant on-time oscillator, the current programming comparator, the feedback transconductance amplifier, and the synchronous rectifier controller. A simplified circuit diagram is shown in Figure 1. OSCILLATOR & COMPARATOR The oscillator has a constant on-time and a minimum offtime. The off-time is extended as long as the output of the current programming comparator is low. Note that in constant on-time control, a discharge (off-time) cycle is needed for the inductor current to be sensed. The minimum off-time is required to account for the finite circuit delays in sensing the inductor output current. TRANSCONDUCTANCE AMPLIFIER The feedback transconductance amplifier generates a current from the voltage difference between the output and the reference. This current produces a voltage across Rgm that adds to the negative voltage on the current sense resistor, RSENSE. When the current level in the inductor drops low enough to cause the voltage at the non-inverting input of the current programming comparator to go positive, the comparator trips and the converter starts a new on-cycle. The current programming comparator controls the length of the off-time by waiting until the current in the secondary decreases to the value specified by the feedback transconductance amplifier. In this way, the feedback transconductance amplifier`s output current steers the current level in the inductor. When the output voltage drops due to a load increase, it will increase the output current of the feedback amplifier and generate a larger voltage across Rgm which in turn raises the secondary current trip level. However, when the output voltage is too high, the feedback amplifier's output current will eventually become negative. Because the output current of the inductor can never go negative by virtue of the diode, the non-inverting input of the comparator will also stay negative. This causes the converter to stop operation until the output voltage drops enough to increase the output current of the feedback transconductance amplifier above zero.
VOUT VIN 4 VFB RP FEEDBACK TRANSCONDUCTANCE AMPLIFIER
+
IS RESR LP 1:1
LOAD
CURRENT PROGRAMMING COMPARATOR
+ -
COMP
CONSTANT ON-TIME MINIMUM OFF-TIME OSCILLATOR ONE SHOT tON 2.5s ONE SHOT tOFF 500ns OUT 1 6
C
CP
VREF
-
Rgm RECTIFIER COMPARATOR
- +
COMP
OUT 2 BLANKING A2 SENSE 7
ML4863
2 RSENSE
Figure 1. Schematic of the ML4863 Controller and Power Stage
5
ML4863
FUNCTIONAL DESCRIPTION
SYNCHRONOUS RECTIFIER CONTROL The control circuitry for the synchronous rectifier does not influence the operation of the main controller. The synchronous rectifier is turned on during the minimum off time, or whenever the SENSE pin is less than -18mV. During transitions where the primary switch is turned on before the voltage on the SENSE pin goes above -18mV, the gate of the synchronous rectifier is discharged softly to avoid accidently triggering the current-mode comparator with the gate discharge spike on the sense resistor. The part will also operate with a Schottky diode in place of the synchronous rectifier, but the conversion efficiency will suffer. CURRENT LIMIT AND MODES OF OPERATION The normal operating range and current limit point are determined by the current programming comparator. They are dependent on the value of the synchronous rectifier current sense resistor (RSENSE), the nominal transformer primary inductance (LP), and the input voltage. RSENSE can be calculated by:
R SENSE = VIN0 MIN5
(Continued) where h = converter efficiency. Once RSENSE has been determined, LP can be found: LP = (25 x 10 -6 ) x VIN0MAX5 x R SENSE (2)
Three operational modes are defined by the voltage at the SENSE pin at the end of the off-time: discontinuous mode, continuous mode, and current limit. The following values can be used to determine the current levels of each mode: VSENSE < 0V: discontinuous mode 0V < VSENSE < 160mV: continuous mode 160mV < VSENSE < 235mV: current limit Inserting the maximum value of VSENSE for each operational mode into the following equation will determine the maximum current levels for each operational mode:
IOUT = VIN V t x VIN x SENSE + ON x VOUT + VIN R SENSE 2 x LP

(3)
VOUT + VIN
150mV V I 0 5 + 20 V 0 0 5 I5
IN MIN OUT MAX IN MAX
OUT MAX
0
h 5
(1)
6
ML4863
DESIGN CONSIDERATIONS
DESIGN PROCEDURE A typical design can be implemented by using the following procedure. 1. Specify the application by defining: The maximum input voltage (VIN(MAX)) The mainimum input voltage (VIN(MIN)) The maximum output current (IOUT(MAX)) The maximum output ripple (DVOUT) As a general design rule, the output ripple should be kept below 100mV to ensure stability. 2. 3a. 3b. Select a sense resistor, RSENSE, using equation 1.
Motorola
See Table 1 for suggested component manufacturers.
Component Manufacturer Sense Resistors Inductors Dale IRC Coilcraft Coiltronics Dale Capacitors MOSFETs AVX Sprague National Part Number LRC Series WSL Series R4999 Phone (402) 563-6506 (512) 992-7900 (847) 639-6400
OCTA-PAC Series (561)241-7876 LPE-6562 Series (605) 665-9301 LPT-4545 series TPS series 593D Series NDS94XX NDS99XX MMDF Series MMSF Series Littlefoot Series (207) 282-5111 (207) 324-4140 (800) 272-9954 (602) 897-5056 (408) 988-8000
Determine the inductance required for the optimum output ripple using equation 2. Determine the minimum inductor current rating required. The peak inductor current is calculated using the following formula:
IL PEAK =
-6 . 235mV VIN ( MAX) (25 10 ) + R SENSE LP
Siliconix
Table 1. Component Suppliers
(4)
DESIGN EXAMPLE 1. Specify the application by defining: VIN(MAX) = 6V VIN(MIN) = 4V IOUT(MAX) = 500mA DVOUT = 100mV Select the sense resistor, RSENSE, using Equation 1: R SENSE = 4 150mV 4V x + x 0.85 5+ 4 500mA 20 x 6 x 0.5
3c.
Specify the inductor's DC winding resistance. A good rule of thumb is to allow 5mW, or less, of resistance per H of inductance. For minimum core loss, choose a high frequency core material such as Kool-Mu, ferrite, or MPP. 2. Specify the coupled inductor's turns ratio: Np : Ns = 1:1
3d.

(1a)
4a.
Calculate the minimum output capacitance required using:
C = IOUT ( MAX)
RSENSE = 138mW @ 120mW 3a. (5) Determine the inductance required using equation 2. LP = (25 x 10 -6 ) x 6 x 0.12 = 18H 3b. (6) IL PEAK 235mV 6 x (25 x 10 -6 ) . = + = 2.79A 120m 18 x 10 -6 (4a) (2a)
V
OUT
+ VIN ( MAX) VOUT
25 10 .DV
-6
OUT
4b.
Establish the maximum allowable ESR for the ouput capacitor: RESR <
DVOUT R SENSE
150mV
Determine the minimum inductor current rating required.
5.
As a final design check, evaluate the system stability (no compensation, single pole response) by using the following equation:
VOUT (6 x 10 -6 ) x
R !
SENSE
x (VOUT + VIN (MIN) ) LP
"# $
(7)
where RSENSE and LP are the actual values to be used.
7
ML4863
DESIGN CONSIDERATIONS (Continued)
3c. Specify the inductor's DC winding resistance: LDCR = 90mW 3d. Specify the coupled inductor's turn ratio: Np : Ns = 1:1 4a. Calculate the minimum output capacitance required using equation 5. LAYOUT Good PC board layout practices will ensure the proper operation of the ML4863. Important layout considerations follow: * The connection from the current sense resistor to the SENSE pin of the ML4863 should be made by a separate trace and connected right at the sense resistor lead. (5a) * The VCC bypass capacitor needs to be located close to the ML4863 for adequate filtering of the IC's internal bias voltage. * Trace lengths from the capacitors to the inductor, and from the inductor to the FET should be as short as possible to minimize noise and ground bounce. (6a) * Power and ground planes must be large enough to handle the current the converter has been designed for. See Figure 5 for a sample PC board layout.
C = 0.50 x
4b.
. 5 + 6 x 25 x 10 5 0.1
-6
= 55F
Establish the maximum ESR for the output capacitor using equation 6. RESR < 0.1x 0.12 = 80mW 150mV
Based on these calculations, the design should use two 100F capacitors, with an ESR of 100mW each, in parallel to meet the capacitance and ESR requirements. 5. As a final design check, evaluate the system stability using equation 7. 100mV (6 x 10 -6 ) x
0.12 x (5 + 4) "# = 360mV (7a) ! 18 x 10 $
-6
Since the inequality is met, the circuit should be stable. Some typical application circuits are shown in Figures 2, 3, and 4.
VIN 47F ML4863 VIN GND
Coiltronics CTX20-4
VOUT 5V, 1A 400F
VIN 100F ML4863
Dale LPE6562
VOUT 5V, 2A 800F
NDS9955
VIN
GND
SENSE OUT 2 SHDN OUT 1 VFB VCC 1F
SENSE OUT 2 SHDN OUT 1 VFB VCC 1F NDS9410 NDS9410
100m
50m
Figure 2. 5V, 1A Circuit
Figure 3. 5V, 2A Circuit
8
ML4863
12V C4 33F 20V C5 33F 20V
5V C6 100F 6.3V T1 DALE LPE-6562-A145 7 1,5 6,10 NDS9955 Q1A Q1B 4 Q2A 2 Q2B MMDF3N03 ML4863 VIN C1 33F 20V C2 33F 20V VIN GND R1 120m 3 C7 100F 6.3V C8 100F 6.3V C9 100F 6.3V
9
8
3.3V C10 100F 6.3V C11 100F 6.3V C12 100F 6.3V C13 100F 6.3V
SENSE OUT 2 SHDN OUT 1 VFB VCC C3 1F 50V R2 30m
SHDN
R3 60m
Figure 4. 5W Multiple Output DC-DC Converter
Figure 5. Typical PC Board Layout
9
ML4863
PHYSICAL DIMENSIONS inches (millimeters)
Package: S08 8-Pin SOIC
0.189 - 0.199 (4.80 - 5.06) 8
PIN 1 ID
0.148 - 0.158 0.228 - 0.244 (3.76 - 4.01) (5.79 - 6.20)
1 0.017 - 0.027 (0.43 - 0.69) (4 PLACES) 0.050 BSC (1.27 BSC) 0.059 - 0.069 (1.49 - 1.75) 0 - 8
0.055 - 0.061 (1.40 - 1.55)
0.012 - 0.020 (0.30 - 0.51) SEATING PLANE
0.004 - 0.010 (0.10 - 0.26)
0.015 - 0.035 (0.38 - 0.89)
0.006 - 0.010 (0.15 - 0.26)
ORDERING INFORMATION
PART NUMBER ML4863CS ML4863ES ML4863IS (Obsolete) TEMPERATURE RANGE 0C to 70C -20C to 70C -40C to 85C PACKAGE 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08)
(c) Micro Linear 1997. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167. Japan: 2,598,946; 2,619,299; 2,704,176. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application.
2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com
DS4863-01
10

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