P

SW(fsw) + VIN

I

O

t

SW

f

SW + 55 V

5 A

20 ns

130 kHz + 0.715 W

P

COND + 1.2

2

0.12

(1 ) 0.007

(150 * 25)) + 0.324 W

I

RMS + IO

d + 5

0.0588 + 1.2 A

DI + IO

2

0.2 + 5

2

0.2 + 2.0 A

f

SW +

0.0588

400 ns

+ 147 kHz

1

T

SW

+ fSW +

V

O(min)

V

IN(max)

T

ON

V

O(min)

V

IN(max)

+

t

ON

T

SW

or

d

MIN +

V

O(min)

V

IN(max)

+ 0.0588

d

MAX +

V

O(max)

V

IN(min)

++ 0.187

TPS40060

TPS40061

SLUS543F – DECEMBER 2002 – REVISED JUNE 2013

www.ti.com

DESIGN EXAMPLE

•

Input voltage: 18 VDC to 55 VDC

•

Output voltage: 3.3 V ±2%

•

Output current: 5 A (maximum, steady-state), 7 A (surge, 10-ms duration, 10% duty cycle maximum)

•

Output ripple: 33 mVP-P at 5 A

•

Output load response: 0.3 V => 10% to 90% step load change

•

Operating temperature: –40°C to 85°C

•

fSW = 130 kHz

1. Calculate maximum and minimum duty cycles

(42)

2. Select switching frequency

The switching frequency is based on the minimum duty cycle ratio and the propagation delay of the current limit

comparator. In order to maintain current limit capability, the on time of the upper MOSFET, tON, must be greater

than 330 ns (see Electrical Characteristics table). Therefore

(43)

(44)

Using 400 ns to provide margin,

(45)

Since the oscillator can vary by 10%, decrease fSW, by 10%

fSW = 0.9 × 147 kHz = 130 kHz

and therefore choose a frequency of 130 kHz.

3. Select

ΔI

In this case

ΔI is chosen so that the converter enters discontinuous mode at 20% of nominal load.

(46)

4. Calculate the high-side MOSFET power losses

Power losses in the high-side MOSFET (Si9407AGY) at 55-VIN where switching losses dominate can be

calculated from Equation 46 through Equation 49.

(47)

substituting Equation 47 into Equation 29 yields

(48)

and from Equation 31, the switching losses can be determined.

(49)

The MOSFET junction temperature can be found by substituting Equation 33 into Equation 32

22

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