L6910ATR datasheet(14/21 Pages) STMICROELECTRONICS

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L6910A L6910

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Figure 12. Demo Board Efficiency @ Vin = 5V

APPLICATION IDEA: 5V TO 12V INPUT; 3.3V / 25A OUTPUT

This is a typical application to replace the mag-amp in the silver box. The output voltage is programmed by the

switch to 3.3V and the maximum output current is of 25A DC. The power mosfet are configured with three

STS11NF30L (30V, 9m

Ω typ @ Vgs = 10V ) for high side and two of them for the low side.

Inductor selection

Since the maximum output current is 25A, to have a 20% ripple (5A) in worst case the inductor chosen is 1.1

µH.

An iron powder core (TO50-52B) with 6 windings has been chosen.

Output Capacitor

4 POSCAP capacitors, model 6TPB330M, are used, with a maximum ESR equal to 40m

Ω each. Therefore the

resultant ESR is of 10m

Ω. For load transient of 20A in the worst case the voltage drop is lower than 5%:

∆Vout = 20 · 0.01 = 200mV

Input Capacitor

For IOUT = 25A and D = 0.5 (worst case for input ripple current), Irms is equal to 12.5A. Three OSCON electro-

lytic capacitors 6SP680M, with a maximum ESR equal to 13m

Ω, are chosen to sustain the ripple. Therefore,

the resultant ESR is equal to 13m

Ω/3 = 4.3mΩ. So the losses in worst case are:

P = ESR ·

= 670mW

Over-Current Protection

The peak current is in this case equal to 30A, substituting the demo board parameters in the relationship report-

ed in the relative section, (IOCSMIN = 170µA; IP = 30A; RDSONMAX = 3mΩ) it results that ROCS = 530Ω.

246

Output Current [A]

Vout = 0.9V

Vout = 1.2V

Vout = 1.5V

Vout = 1.8V

Vout = 2.5V

Vout = 3.3V

rm s

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