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SP7662
TM
Evaluation Board Manual
Easy Evaluation of the SP7662ER 0 to 12A Output Synchronous Buck Converter Built in Low RDS(ON) Power FETs UVLO Detects Both VCC and VIN Highly Integrated Design, Minimal Components High Efficiency: up to 93% Feature Rich: UVIN, Programmable Soft Start, Built In VCC Supply Output Short Circuit Protection and Current Limiting
SP7662EB Schematic
Nov27-06
SP7662 Evaluation Board Manual
(c) 2006 Sipex Corporation
Using the Evaluation Board 1) Powering Up the SP7662EB Circuit Connect the SP7662 Evaluation Board with an external +12V power supply. Connect with short leads and large diameter wire directly to the "VIN" and "GND" posts. Connect a Load between the VOUT and GND posts, again using short leads with large diameter wire to minimize inductance and voltage drops. Provide >=100lfm of airflow if powering the evaluation board beyond 10 Amps for extended periods. 2) Measuring Output Load Characteristics VOUT ripple can best be seen by touching the probe tip to the pad for C5 and the scope GND collar touching the Output GND post - avoid a GND lead on the scope which will increase noise pickup. 3) Using the Evaluation Board with Different Output Voltages While the SP7662 Evaluation Board has been tested and delivered with the output set to 3.30V, by simply changing one resistor, R2, the SP7662 can be set to other output voltages. The relationship in Equation 1 is based on a voltage divider from the output to the feedback pin VFB, which is set to an internal reference voltage of 0.80V. Note, due to the common mode voltage range of the current sense amplifier, output voltages greater than 3.3V are only possible if the current sense is disabled. To disable current limit, remove R3 and R4. Standard 1% metal film resistors of surface mount size 0603 are recommended.
R2 = R1 Vout .80V - 1
Equation 1
Where R1 = 10k and for VOUT = 0.80V setting, simply remove R2 from the board. Furthermore, one could select the value of the R1 and R2 combination to meet the exact output voltage setting by restricting R1 resistance range such that 10K R1 100K for overall system loop stability. Note that since the SP7662 Evaluation Board design was optimized for 12V down conversion to 3.30V, changes of output voltage and/or input voltage will alter performance from the data given in the Power Supply Data section. In addition, the SP7662ER provides short circuit protection by sensing VOUT at GND. The current limit of the converter is set to about 17A which is accomplished by sensing the current through the inductor. To adjust the current limit, follow Equations 2 and 3 to set the current limit accordingly. The current limit should be set to about 50% higher than the maximum output current that is desired. This will prevent the part from accidentally triggering the current limit during large transient load steps. Adjusting the Current upward is done by adjusting resistor R9.
R9 =
Where
60mV ( R3 + R4) I max ( DCR - 60mV)
Equation 2
DCR is the Inductor winding resistance IMAX is the desired output current
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(c)2007 Sipex Corporation
Adjusting the Current downward is controlled by adjusting R8.
(Vout - 60mV) + (Imax DCR) R8 = R4 60mV - (Imax DCR)
Where DCR is the Inductor winding resistance IMAX is the desired output current
Equation 3
Further details on the current limit can be found in the SP7662 data sheet. For the Inter-Technical Inductor on this demo board DCR= 4.1m nominal. POWER SUPPLY DATA The SP7662ER is designed with an very accurate 2.0% reference over line, load and temperature. Figure 1 data shows a typical SP7662 evaluation board efficiency plot, with efficiencies up to 93% and output currents up to 12A. The output voltage ripple of less than 30mV at full load and the LX node are shown in figure 2. Figures 3 and 4 illustrate a 6A-to-12A and 0A-to-12A Load Step. Short circuit and current limit are shown in Figures 5 and 6. Typical startup characteristics into a full load and no load are shown in figures 7 and 8. All data was taken at 12VIN. While data on individual power supply boards may vary, the capability of the SP7662ER of achieving high accuracy over a range of load conditions shown here is quite impressive and desirable for accurate power supply design.
7662 Efficiency 12Vin - 3.3Vout - 12Amps
96.00 94.00 92.00 90.00
Efficiency
88.00 86.00 84.00 82.00 80.00 78.00 76.00 0 2 4 6 8 10 12
Iout (Amps)
Figure 1. Efficiency vs Load
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Figure 2. LX node output ripple voltage
Figure 3. Load Step Response: 6->12A
Figure 4. Load Step Response 0->12A
Figure 5. Current limit set point 17A
Figure 6. Output load Short Circuit
Figure 7. Startup into full load
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Figure 8. Startup into no load
TYPE III LOOP COMPENSATION DESIGN The open loop gain of the SP7662EB can be divided into the gain of the error amplifier Gamp(s), PWM modulator Gpwm, buck converter output stage Gout(s), and feedback resistor divider Gfbk. In order to cross over at the selecting frequency fco, the gain of the error amplifier must compensate for the attenuation caused by the rest of the loop at this frequency. The goal of loop compensation is to manipulate the open loop frequency response such that its gain crosses over 0dB at a slope of -20dB/dec. The open loop crossover frequency should be higher than the ESR zero of the output capacitors but less than 1/5 to 1/10 of the switching frequency fs to insure proper operation. Since the SP7662EB is designed with Ceramic Type output capacitors, a Type III compensation circuit is required to give a phase boost of 180 in order to counteract the effects of the output LC underdamped resonance double pole frequency.
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(c)2007 Sipex Corporation
Figure 9. Voltage Mode Control Loop with Loop Dynamic for Type III Compensation The simple guidelines for positioning the poles and zeros and for calculating the component values for Type III compensation are as follows. As a particular example, consider for the following SP7662EB with a Type III Voltage Loop Compensation component selections: Input requirements and inductor selection VIN = 12V, we will use 12V as VIN max for this design VOUT = 3.30V @ 0 to 12A load Select L = 2.7uH => yield 25% of maximum 12A output current ripple Select COUT = 100uF Ceramic capacitor (RESR 3m) fs = 300kHz SP7662 internal Oscillator Frequency VRAMP_PP = 1.0V SP7662 internal Ramp Peak-to-Peak Amplitude
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(c)2007 Sipex Corporation
Step by step design procedures:
Note: Loop Compensation component calculations discussed in this section can be quickly iterated with the Type III Loop Compensation Calculator on the web at: www.sipex.com/files/Application-Notes/TypeIIICalculator.xls
Choose
fco = fs/10
fco = 300kHz/10 = 30kHz
Calculate fp_LC the double pole frequency of the filter
fp_LC =
1 2 ( L C )
1 2 2.7uH 200uF ) = 6852hz 7kHz
fp_LC =
Calculate fz_ESR the ESR zero frequency
fz_ESR =
1 2 Cesr Cout
fz _ ESR =
1 = 530 kHz 2 (3m / 2) (200 F )
Select R1 component value such that 10k R1 100k R1 = 10k, 1% Calculate R2 based on the desired VOUT
R2 =
R1 Vout .8V - 1
68.1K = 3200 3160 3.3V .8V - 1
R2 =
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(c)2007 Sipex Corporation
Select the ratio of RZ2 / R1 gain for the desired gain bandwidth (we will use slightly higher than 30kHz @ 33kHz) Vramp _ pp fco RZ 2 = R1 Vin _ max fp _ LC
1V 33kHz RZ 2 = R1 = 3928 4020 12V 7 kHz
Calculate CZ2 by placing the zero at 1/2 of the output filter pole frequency
CZ 2 =
1 RZ 2 fp _ LC
CZ 2 =
1 = 11nF 10nF 4020 7kHz
Calculate CP1 by placing the first pole at ESR zero frequency
CP1 =
1 2 ( Rz 2 fz _ ESR )
1 = 133 pF 100 pF 2 (4020 530 KHz )
CP1 =
Calculate RZ3 by setting the second pole at 1/2 of the switching frequency and the second zero at the output filter double pole frequency
RZ 3 =
2 ( R1) ( fp _ LC ) fs - 2 fp _ LC
2 (10k) (7 kHz ) = 489 453 300 KHz - 2 7 kHz
RZ 3 =
Calculate CZ3 from RZ3 component value above
CZ 3 =
1 RZ 3 fs
CZ 3 =
1 = 2.3nF 2200 pF 453 300 KHz
Choose 100pF CF1 220pF to stabilize the SP7662ER internal Error Amplifier . For this example let's select 100pF.
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(c)2007 Sipex Corporation
PC Layout Drawings
Figure 15. SP7662EB Component Placement
Figure 16. SP7662EB PC Layout Top Side
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Figure 17. SP7662EB PC Layout 2nd Layer Side
Figure 18. SP7662EB PC Layout 3rd Layer Side
Figure 19. SP7662EB PC Layout Bottom Side
Figure 20. SP7662EB PC Layout Bottom Side Components
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(c)2007 Sipex Corporation
Table 1: SP7662EB Suggested Components and Vendor Lists
ORDERING INFORMATION Model Temperature Range Package Type SP7662EB...................................-40C to +85C...............................SP7662 Evaluation Board SP7662ER.................................. -40C to +85C.........................................26-pin DFN(option2)
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(c)2007 Sipex Corporation

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