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APU3146

DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND AUTO-RESTART

Advanced Power Electronics Corp 

APU3146

Input Capacitor Selection

The 1800 out of phase will reduce the RMS value of the

ripple current seen by input capacitors. This reduces

numbers of input capacitors. The input capacitors must

be selected that can handle both the maximum ripple

RMS at highest ambient temperature as well as the

maximum input voltage. The RMS value of current ripple

for duty cycles under 50% is expressed by:

IRMS= (I12D1(1-D1)+I22D2(1-D2)-2I1I2D1D2) --- (6)

Where:

IRMS is the RMS value of the input capacitor current

D1 and D2 are the duty cycle for each output

I1 and I2 are the current for each output

For this application the IRMS =4.8A

For higher efficiency, low ESR capacitors is recom-

mended.

Choose two Poscap from Sanyo 16TPB47M (16V, 47µF,

70mΩ ) with a maximum allowable ripple current of 1.4A

for inputs of each channel.

Inductor Selection

The inductor is selected based on operating frequency,

transient performance and allowable output voltage ripple.

Low inductor value results to faster response to step

load (high ∆i/∆t) and smaller size but will cause larger

output ripple due to increase of inductor ripple current.

As a rule of thumb, select an inductor that produces a

ripple current of 10-40% of full load DC.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

VIN - VOUT = L×∆∆ti

;

∆t

=

D×

1

fS

L = (VIN - VOUT)×VIN×VO∆UiT×fS

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

;

D

=

VOUT

VIN

---(7)

For ∆i(2.5V) = 38%(IO(2.5V) ), then the output inductor will

be:

L4 = 1.71µH

For ∆i(1.8V) = 30%(IO(1.8V) ), then the output inductor will

be:

L3 = 1.7µH

Panasonic provides a range of inductors in different val-

ues and low profile for large currents.

Choose ETQP6F1R8BFA (1.71µH, 14A, 3.3mΩ) both

for L3 and L4.

For 2-phase application, equation (7) can be used for

calculating the inductors value. In such case the induc-

tor ripple current is usually chosen to be between 10-

40% of maximum phase current.

Output Capacitor Selection

The criteria to select the output capacitor is normally

based on the value of the Effective Series Resistance

(ESR). In general, the output capacitor must have low

enough ESR to meet output ripple and load transient

requirements, yet have high enough ESR to satisfy sta-

bility requirements. The ESR of the output capacitor is

calculated by the following relationship:

(ESL, Equivalent Series Inductance is neglected)

ESR

≤

∆VO

∆IO

---(8)

Where:

∆VO = Output Voltage Ripple

∆i = Inductor Ripple Current

∆VO = 3% of VO will result to ESR(2.5V) =19.7mΩ and

ESR(1.8V) =16mΩ

The Sanyo TPC series, Poscap capacitor is a good choice.

The 6TPC330M, 330µF, 6.3V has an ESR 40mΩ. Se-

lecting two of these capacitors in parallel for 2.5V out-

put, results to an ESR of ≅ 20mΩ which achieves our

low ESR goal. And selecting four of these capacitors in

parallel for 1.8V output, results to an ESR of ≅ 10mΩ

which achieves our low ESR goal.

The capacitors value must be high enough to absorb the

inductor's ripple current.

Power MOSFET Selection

The APU3146 uses four N-Channel MOSFETs. The se-

lections criteria to meet power transfer requirements is

based on maximum drain-source voltage (VDSS), gate-

source drive voltage (VGS), maximum output current, On-

resistance RDS(ON) and thermal management.

The both control and synchronous MOSFETs must have

a maximum operating voltage (VDSS) that exceeds the

maximum input voltage (VIN).

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