REFERENCE DESIGN OF 15A POL Using IR3447 FOR ......C8 2200pF U3 Vin IR3447 17 LGnd 11 Rt/Sync 4 SW...

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This evaluation board is a preliminary version meant for the engineering evaluation of the IR3447. Based on the results of the continuing evaluation, this board can evolve and change without notice

4/1/20131

REFERENCE DESIGN OF 15A POL Using IR3447 FOR HUAWEI CBB PROJECT

DESCRIPTION

The IR3447 is a synchronous buckconverter, providing a compact, highperformance and flexible solution in a small5mmx6mm QFN package.

Key features offered by the IR3447 includeinternal Digital Soft Start, precision 0.6Vreference voltage, Power Good, thermalprotection, programmable switchingfrequency, Enable input, input under-voltagelockout for proper start-up, enhanced line/load regulation with feed forward, externalfrequency synchronization with smoothclocking, internal LDO, true differentialremote sensing and pre-bias start-up.

A thermally compensated output over-currentprotection function is implemented by sensingthe voltage developed across the on-resistanceof the synchronous rectifier MOSFET foroptimum cost and performance.

This document provides the designrecommendation for the feedback loopcompensation, input and output capacitors tomeet Huawei’s applications. The design istested on a standard IR3447 demo board toverify the loop stability and input/output voltageripple. Switching node spike, efficiency andthermal performance are also tested forcustomer’s reference.

APPLICATION INFORMATION

• Vin = 6V-19V (12V, Nominal), No Vcc required.

• Vout = 0.9V @ 0-15A/ [email protected], Refer to Table 1 for details

• Fs=300kHz ~ 350kHz

• L= 0.6uH, Pulse PG0702 601NL, 0.91mΩ, Isat = 32A

• Input/Output voltage ripple = 1% of Vin/Vout

• Transient load response = 3% of Vout @ 50% step load current

• Efficiency >90% @ Vin = 12V, Vout = 0.9V, Io=15A

• Max ambient temperature: 100ºC, 0.5m/s

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4/1/20132

APPLICATION INFORMATION

Feedback Loop Compensation DesignThe IR3447 employs voltage mode control with a proprietary PWM scheme, allowing the closed loopbandwidth as high as 1/5th of the switching frequency. The input voltage feedforward ensures the cross-over frequency independent of the input voltage, which simplifies the design effort. Please note that asVin is approaching to 6V, the performance of input voltage feedforward may be degraded slightly dueto the nonlinearity of the internal circuit. As a result, the cross-over frequency is lower at Vin of 6V.

The loop compensation is designed for a 60kHz cross-over frequency, with a phase margin higher than50º and gain margin larger than 10dB to accommodate the component variations and noise/jittering.Since SP caps are used for the output capacitors, type III-A compensation is used. Please refer to theapplication note AN-1162 “Compensator Design Procedure for Buck Converter with Voltage-ModeError-Amplifier “ for more information. SupIRBuck excel design tool is a very useful tool to create thedesign without going through the complicated math calculations.

Table I. Vout vs Iomax

Vout (V) Iomax (A)

0.9V 15A

1.0V 10A

1.15V 7A

1.5V 9A

1.8V 5.5A

Output Capacitor SelectionThe selection of the output capacitors are determined by the requirements of the output voltage rippleand transient load response. For this application, the low switching frequency limits the highestachievable control bandwidth. To meet the transient response requirement, two SP capacitors,470uF/2.5V/6mΩ (Panasonic EEFSX0E471XE), are used. Please note that EEFSX0E471XE is ratedfor -40ºC~105ºC operating temperature. Voltage derating may be required depending on the operatingtemperature over 105ºC. The capacitors rated for 125ºC operating temperature may be used for betterlong-term reliability.

Additional ceramic capacitors are needed to meet the output voltage ripple requirement. For the benchtest, ten ceramic capacitors, 47uF/X5R/6.3V/0805 (TDK C2012X5R0J476M125AC), are used alongwith the SP capacitors. However, for 100ºC ambient operating temperature, the capacitors with X7Rtemperature coefficient should be selected (eg.TDK CGA5L1X7R0J226M160AC). It should be pointedout that the capacitance value changes significantly with the DC bias voltage. This parameter alsovaries a lot among different capacitor vendors. It is important to check the DC-bias characteristics in thedatasheet. In addition, the AC ripple voltage also has some effect on the capacitance.

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Switch Node Spike

The switching node voltage spike can be reduced by a good PCB layout design, such as minimize theloop inductance, placing the input capacitors close to the input and GND pads. In addition, adding aboot resistor can help to slow down the switching speed and therefore to reduce the switch node spike,at the expense of the reduction of the efficiency.

For this application, the max input voltage is 19V. To ensure a reliable operation, it is recommended touse a 2Ω resistor in series with the boot capacitor. The max switch node voltage is reduced to 25.5Vfrom 31V. The resulting effect on the efficiency is negligible. Please refer to test results for moreinformation.

Input Capacitor Selection

The selection of the input capacitors are determined by the requirement of the input voltage ripple.Ceramic capacitors are selected for its low ESR and ESL. At the minimum input voltage, the input ripplecurrent is maximum. At the maximum input voltage, the capacitance value decreases most due to theDC bias. So the input voltage ripple should be tested under these two conditions. For the bench test,ten ceramic capacitors, 22uF/X5R/25V/1206 (Murata GRM31CR61E226KE15L), are used. However, for100ºC ambient operating temperature, the capacitors with X7R temperature coefficient should beselected (e.g. 10uF/X7R/25V/1206, TDK C3216X7R1E106K160AB).

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4/1/20134

Fig

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R32=R3=4.53k, Vo=1.5V

R32=R3=3.40k, Vo=1.8V

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Bench test used: 10x 22uF/X5R/25V/1206 (Murata GRM31CR61E226KE15L)

For 100C operating ambient temp, select capacitors with X7R temperature coefficient.

Bench test used: 10x 47uF/X5R/6.3V/0805 (TDK C2012X5R0J476M125AC)

For 100C operating ambient temp, select capacitors with X7R temperature coefficient.

R35

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IRDC3447-P0V9/P1V8

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4/1/20135

TYPICAL OPERATING WAVEFORMSVin=6V-19V, Vo=0.9V, Io=0-15A, 300kHz, Room Temperature, no airflow

Vout Ripple Vin=12V, Vo=0.9V, Io=15A Ch2:Vin, Ch3:Vo, Vo-ripple=9.2mV



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Step Load Response, Vin=12V, Vo=0.9V, Io=7.5A-0A-7.5A, Ch3:Vout, Ch4:Io, Overshoot=12.6mV, Undershoot=-22.4mV

Step Load Response, Vin=12V, Vo=0.9V, Io=15A-7.5A-15A, Ch3:Vout, Ch4:Io, Overshoot=13.6mV, Undershoot=-24mV


Step Load Response, Vin=6V, Vo=0.9V, Io=15A-7.5A-15A, Ch3:Vout, Ch4:Io, Overshoot=18.4mV, Undershoot=-27.2mV


Step Load Response, Vin=19V, Vo=0.9V, Io=15A-7.5A-15A, Ch3:Vout, Ch4:Io, Overshoot=13.6mV, Undershoot=-22.4mV

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Vin Ripple Vin=12V, Vo=0.9V, Io=15A Ch3:Vin, Vin-ripple=92mV

Vin Ripple Vin=19V, Vo=0.9V, Io=15A Ch3:Vin, Vin-ripple=104mV

Vin Ripple Vin=6V, Vo=0.9V, Io=15A Ch3:Vin, Vin-ripple=100.0mV

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4/1/20138


SW Node Voltage Vin=12V, Vo=0.9V, Io=15A, Rboot=2ΩCh3:Vsw, Vsw=19.7V max, -3.1V min




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Bode Plot: Vin=12V, Vout = 0.9V and 15A load: bandwidth of 59 kHz; PM= 54º, GM=-15dB


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Power loss versus load current

Efficiency versus load current


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Vin=12V, Vo=0.9V, Io=15A, no airflow, room temperature, IR3447=51º, Inductor = 49º, Board=39º


Vin=6V, Vo=0.9V, Io=15A, no airflow, room temperature, IR3447=45º, Inductor = 44º, Board=36º

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4/1/201313


Vout Ripple Vin=12V, Vo=1.8V, Io=5.5A Ch2:Vin, Ch3:Vo, Vo-ripple=17.2mV

Vout Ripple Vin=19V, Vo=1.8V, Io=5.5A Ch2:Vin, Ch3:Vo, Vo-ripple=18mV

Vout Ripple Vin=6V, Vo=1.8V, Io=5.5A Ch2:Vin, Ch3:Vo, Vo-ripple=16mV

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4/1/201314

TYPICAL OPERATING WAVEFORMSVin=6V-19V, Vo=1.8V, Io=0-5.5A, 300kHz, Room Temperature, no airflow


Step Load Response, Vin=12V, Vo=1.8V, Io=5.5A-2.5A-5.5A, Ch3:Vout, Ch4:Io, Overshoot=14.4mV, Undershoot=-18.4mV


Step Load Response, Vin=6V, Vo=1.8V, Io=5.5A-2.5A-5.5A, Ch3:Vout, Ch4:Io, Overshoot=15.2mV, Undershoot=-18.4mV


Step Load Response, Vin=19V, Vo=1.8V, Io=5.5A-2.5A-5.5A, Ch3:Vout, Ch4:Io, Overshoot=16mV, Undershoot=-20mV

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4/1/201315

TYPICAL OPERATING WAVEFORMSVin=6V-19V, Vo=1.8V, Io=0-5.5A, 300kHz, Room Temperature, no airflow

Bode Plot: Vin=12V, Vout = 1.8V and 5.5A load: bandwidth of 53 kHz; PM= 60º, GM=-15dB


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4/1/201316



REFERENCE DESIGN OF 15A POL Using IR3447 FOR ......C8 2200pF U3 Vin IR3447 17 LGnd 11 Rt/Sync 4 SW...

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