12

Rev. 2.1

08/20/02

IRU3007

www.irf.com

T

≡ Switching Period

D

≡ Duty Cycle

Vsw

≡ High-side MOSFET ON Voltage

≡ MOSFET On-Resistance

Vsync

≡ Synchronous MOSFET ON Voltage

DIr

≡ Inductor Ripple Current

DVo

≡Output Ripple Voltage

T = 1 / 200000 = 5

ms

Vsw = Vsync = 14.2

30.019 = 0.27V

D

≈ (2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61

35 = 3.1ms

ms

DIr = (2.8 + 0.27)31.9 / 3 = 1.94A

DVo = 1.9430.006 = 0.011V = 11mV

Vf = 0.5V

≈ (3.3 + 0.5) / (4.75 - 0.27 + 0.5) = 0.76

of the 1500

mF, 6MV1500GX type Sanyo capacitors. With

Rs=5m

V, the maximum ESR becomes 9.5mV which is

equivalent to

≈ 4 caps. Another important consideration

is that if a trace is being used to implement the resistor,

the power dissipated by the trace increases the case

temperature of the output capacitors which could seri-

ously affect the life span of the output capacitors.

Output Inductor Selection

The output inductance must be selected such that un-

der low line and the maximum output voltage condition,

the inductor current slope times the output capacitor

ESR is ramping up faster than the capacitor voltage is

drooping during a load current step. However, if the in-

ductor is made too small, the output ripple current and

ripple voltage will become too large. One solution to bring

the ripple current down is to increase the switching fre-

quency, however that will be at the cost of reduced effi-

ciency and higher system cost. The following set of for-

mulas are derived to achieve optimum performance with-

out many design iterations.

The maximum output inductance is calculated using the

following equation:

Where:

For Vo = 2.8V and

DI = 14.2A, we get:

Assuming that the programmed switching frequency is

set at 200KHz, an inductor is designed using the

Micrometals’ powder iron core material. The summary

of the design is outlined below:

The selected core material is Powder Iron, the selected

core is T50-52D from Micro Metal wound with 8 turns of

#16 AWG wire, resulting in 3

mH inductance with

≈ 3 mV

of DC resistance.

Assuming L=3

mH and Fsw=200KHz (switching fre-

quency), the inductor ripple current and the output ripple

voltage is calculated using the following set of equations:

L = ESR

3 C 3

(2

3 DI)

(4.75 - 2.8)

(2

3 14.2)

L = 0.006

3 9000 3

= 3.7

mH

In our example for Vo = 2.8V and 14.2 A load, assuming

IRL3103 MOSFET for both switches with maximum on

resistance of 19m

V, we have:

Power Component Selection

Vcore

Assuming IRL3103 MOSFETs as power components,

we will calculate the maximum power dissipation as fol-

lows:

For high side switch the maximum power dissipation

happens at maximum Vo and maximum duty cycle.

8C

For synch MOSFET, maximum power dissipation hap-

pens at minimum Vo and minimum duty cycle.

3.3V Supply

Again, for high side switch the maximum power dissipa-

tion happens at maximum Vo and maximum duty cycle.

The duty cycle equation for non synchronous replaces

the forward voltage of the diode with the Synch MOSFET

on voltage. In equations below:

T = 1 / Fsw

Vsw = Vsync = Io

D

3T

DVo = DIr3ESR

≈ (2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65

≈ (2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43