Sipex SP7651 User Manual

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Evaluation Board Manual

TM

 Easy Evaluation for the SP7651ER 12V Input, 0 to 3A Output Synchronous Buck Converter

 Built in Low RDS(ON) Power FETs

 UVLO Detects Both Vcc and V

 Highly Integrated Design, Minimal

Components

 High Efficiency: 88%

 Feature Rich: UVIN, Programmable

Softstart, External Vcc Supply and Output Dead Short Circuit Shutdown Protection

IN

SP7651

SP7651EB SCHEMATIC

USING THE EVALUATION BOARD

impressive and desirable for accurate power supply design.

1) Powering Up the SP7651EB Circuit

Connect the SP7651 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 GND2 posts, again using short leads with large diameter wire to minimize inductance and voltage drops.

2) Measuring Output Load Characteristics

V

OUT

ripple can best be seen touching the probe tip to the pad for C3 and the scope GND collar touching the GND side of C3 using short wrapped wire around the collar – avoid a GND lead on the scope which will increase noise pickup.

3) Using the Evaluation Board with Different Output Voltages While the SP7651 Evaluation Board has been tested and delivered with the output set to 3.30V, by simply changing one resistor, R2, the SP7651 can be set to other output voltages. The relationship in the following formula 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. Standard 1% metal film resistors of surface mount size 0603 are recommended.

V

OUT

= 0.80V (R1 / R2 + 1) => R2 = R1 / [ ( Vout / 0.80V ) – 1 ]

Where R1 = 68.1KΩ and for V

OUT

= 0.80V setting, simply remove R2 from the board.

Furthermore, one could select the value of the R1 & R2 combination to meet the exact output voltage setting by restricting R1 resistance range such that 50KΩ ≤ R1 ≤ 100KΩ for overall system loop stability.

Note that since the SP7651 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 SP7651ER provides short circuit protection by sensing V

OUT

at GND.

POWER SUPPLY DATA

The SP7651ER is designed with a very accurate 1.0% reference over line, load and temperature. Figure 1 data shows a typical SP7655 Evaluation Board Efficiency plot, with efficiencies to 87% (including generation of 5V Vcc) and output currents to 3A. SP7651ER Load Regulation is shown in Figure 2 to have only 0.5% change in output voltage from 0.5A load to 3A load. Figures 3 and 4 illustrate a 1.5A to 3A and 0A to 3A Load Step. Start-up Responses in Figures 5, 6 and 7 show a controlled start-up with different output load behavior when power is applied where the input current rises smoothly as the Softstart ramp increases. In Figure 8 the SP7651ER is configured for hiccup mode in response to an output dead short circuit condition and will Soft-start until the over-load is removed. Figure 9 and 10 show output voltage ripple less than 25mV at no load to 3A load.

While data on individual power supply boards may vary, the capability of the SP7651ER of achieving high accuracy over a range of load conditions shown here is quite

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100

Vout

Iout (2A/div)

Iout (1A/div)

Vout

Efficiency (%)

Efficiency vs Load Current

90

80

70

60

50

40

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Load current (A)

Vi=12V Vo=3.3V

Figure 1. Efficiency vs Load

Output Voltage vs Load Current

3.320

3.310

3.300

3.290

Output Voltage (V)

3.280

Vi=12V Vo=3.3V

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Load current (A)

Figure 2. Load Regulation

Vout

Vi=12V Vo=3.3V

Figure 3. Load Step Response: 1.5->3A

Vin

Vi=12V Vo=3.3V

SoftStart

Vout

Vi=12V Vo=3.3V

Figure 4. Load Step Response: 0->3A

Vin

Vi=12V Vo=3.3V

SoftStart

Ichoke (0.5A/div)

Figure 5. Start-Up Response: No Load

Ichoke (2A/div)

Figure 6. Start-Up Response: 3A Load

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SoftStart

Vout

Ichoke 25A/div

SoftStart

Vi=12V Vo=3.3V

Ichoke 25A/div

Figure 7. Output Load Short Circuit Figure 8. Output Load Short Circuit (Zoom-in)

Vout ripple = 10mV

Vi=12V Vo=3.3V

Ichoke (1A/div)

Vout ripple = 10mV

Ichoke (2A/div)

Vi=12V Vo=3.3V

Figure 9. Output Ripple: No Load Figure 10. Output Ripple:3A Load

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TYPE III LOOP COMPENSATION DESIGN

(SRz2Cz2+1)(SR1Cz

SR1Cz2(SRz3Cz3+1)(SRz2C

Vin

Vramp_p

(SResrCout+

[S^2LCout+S(Resr+Rdc)Co

(R1+R2

R2

OR

Vout

Vref

Vout

Vref

Type II

I Voltage

Vfbk

Gout(S)

Gain Block

Compensation Gamp(S)

Output Stage

Gain Block

Gpwm

Gain Block

Gfbk

Voltage Feedback

Resr := Output Capacitor Equivalent Series Resitance

Definition

Rdc := Output

Inductor DC Resistance

Vramp_pp := SP7651 Internal RAMP Amplitude Peak to Peak Voltage

Output Load Resistance >> Resr and Rdc

Cz2 >> Cp1 and R1 >> Rz3

Condition

The open loop gain of the SP7651EB 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 SP7651EB 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.

PWM Stage

(Volts

Figure 11. Voltage Mode Control Loop with Loop Dynamic for Type III Compensation

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The simple guidelines for positioning the poles and zeros and for calculating the component values for Type III compensation are as follows:

a. Choose fco = fs / 5

b. Calculate fp_LC

fp_LC = 1 / [2π (L ●C) ^ 1/2 ]

c. Calculate fz_ESR

fz_ESR = 1 / 2π (R

ESR

) ● (C

OUT

)

d. Select R1 component value such that 50kΩ ≤ R1 ≤ 100kΩ

e. Calculate R2 base on the desired V

R2 = R1 / [(V

OUT

/ 0.80V) – 1]

OUT

f. Select the ratio of Rz2 / R1 gain for the desired gain bandwidth

Rz2 = R1 (V

RAMP_PP

/ VIN_

MAX

) (fco / fp_LC)

g. Calculate Cz2 by placing the zero at ½ of the output filter pole frequency

Cz2 = 1 / [π (Rz2) ● (fp_LC) ]

h. Calculate Cp1 by placing the first pole at ESR zero frequency

Cp1 = 1 / [2π (Rz2) ● (fz_ESR) ]

i. Calculate Rz3 by setting the second pole at ½ of the switching frequency and the

second zero at the output filter double pole frequency Rz3 = 2 (R1) (fp_LC) / fs

j. Calculate Cz3 from Rz3 component value above

Cz3 = 1 / π (Rz3) ● (fs)

k. Choose 100pF ≤ Cf1 ≤ 220pF to stabilize the SP7651ER internal Error Amplifier

Note: Loop Compensation component calculations discussed in this section are further elaborated in the application note #ANP16, “Loop Compensation of Voltage-Mode Buck Converters”.

These calculations shown here can be quickly iterated with the Type III Loop Compensation Calculator on the web at:

www.sipex.com/files/Application-Notes/TypeIIICalculator.xls

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PCB LAYOUT DRAWINGS

Figure 12. SP7651EB Layout Top Side & Component Placement

Figure 13. SP7651EB PC Layout Bottom Side & Component Placement

Figure 14. SP7651EB PC Layout Inner Layer 1

Figure 15. SP7651EB PC Layout Inner Layer 2

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Table 1: SP7651EB List of Materials

REF. DES. U1 1 SIPEX SP7651ER DFN-26 2-FETs Buck Ctrl U2 1 Sipex SPX5205M5-5.0 DBST 1 Diodes Inc. SD101AWS-7 L1 1 Intertechnical SC7232-2R2M 7 X 7mm Inductor, 2.2uH, 8A, 10.4mohm C3 1 TDK CORPORATION C3225X5R0J226M 1210 Capacitor, Ceramic, 22uF, 6.3V, X5R, 20% C4 1 TDK CORPORATION C3225X5R1C226M 1210 Capacitor, Ceramic, 22uF, 16V, X5R, 20% CVCC 1 TDK CORPORATION C1608X5R1A225K 0603 Capacitor, Ceramic, 2.2uF, 10V, X5R, 10% C2 1 TDK CORPORATION C1608X7R1H104K 0603 Capacitor, Ceramic, 0.1uF, 50V, X7R, 10% CBST 1 AVX CORPORATION 06035C682KAT2A 0603 Capacitor, Ceramic, 6.8nF, 50V, X7R, 10% CSS 1 ROHM MCH185CN153KK 0603 Capacitor, Ceramic, 15nF, 50V, X7R, 10% CP1 1 AVX CORPORATION 06035A100JAT2A 0603 Capacitor, Ceramic, 10pF, 50V, C0G, 5% CZ2 1 AVX CORPORATION 06035A391JAT2A 0603 Capacitor, Ceramic, 390pF, 50V, C0G, 5% CZ3 1 AVX CORPORATION 06035A101JAT2A 0603 Capacitor, Ceramic, 100pF, 50V, C0G, 5% CF1 1 ROHM MCH185A101JK 0603 Capacitor, Ceramic, 100pF, 50V, C0G, 5% RZ2 1 VISHAY DALE CRCW0603-1502FRT1 0603 Resistor, 15K, 1/16W, 1% R2 1 SEI ELECTRONICS RMC-1/16W-21.5K-1%-TR 0603 Resistor, 21.5K, 1/16W, 1% RZ3 1 VISHAY DALE CRCW0603-2001FRT1 0603 Resistor, 2K, 1/16W, 1% R1 1 VISHAY DALE CRCW0603-6812FRT1 0603 Resistor, 68.1K, 1/16W, 1% R3 1 VISHAY DALE CRCW0603-2003FRT1 0603 Resistor, 200K, 1/16W, 1% R4 1 VISHAY DALE CRCW0603-1003FRT1 0603 Resistor, 100K, 1/16W, 1% RBST 1 ROHM MCR03EZPEFX20R0 0603 Resistor, 20, 1/16W, 1%

VIN,VOUT, GND,GND2

MANUFACTURER

QTY

NAME Manufacturer Part Number SIZE DESCRIPTION

SOT-23-5

SOD-323

4 Vector Electronic K24C/M .042 Dia Input/Output Terminal Posts

150mA LDO Voltage Regulator Schottky Diode, 15mA

ORDERING INFORMATION

Model Temperature Range Package Type

SP7651EB…................................-40°C to +85°C...............…SP7651 Evaluation Board

SP7651ER..............................…. -40°C to +85°C.................................……26-pin DFN

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