UCC3588

5BIT PROGRAMMABLE OUTPUT BiCMOS

POWER SUPPLY CONTROLLER

SLUS311A – JULY 1999 – REVISED AUGUST 2000

7

POST OFFICE BOX 655303

APPLICATION INFORMATION

2) To properly approximate the full load duty cycle operating range, assumptions are made regarding the

0.014

Ω. The output inductor is allowed to dissipate one watt under full load, giving a dc resistance of 6.9 mW,

and R6 is 3 m

Ω. The resulting duty cycle at the operating extremes is then:

d

MIN +

V

OUT IO )

I

OUT

R6

) R

DS on )

R

V

IN HI

+

1.8

) (12

0.024)

5.5

+ 0.379

d

MIN +

V

OUT HI )

I

OUT

R6

) R

DS on )

R

V

IN LO

+

3.5

) (12

0.024)

4.5

+ 0.842

3) The value of the output inductor is chosen at the worst case ripple current point.

L

+

V

IN HI *

V

OUT LO

DV

OUT

+

(5.5

* 1.8)

0.379

3.333

m

2.4

+ 1.9 mH

Four turns of #16 on a micrometals T51-52C core has an inductance of 1.9

µH, has a dc resistance of 6.6 mΩ,

and will dissipate about 1 W under full load conditions. With an output inductor value of 1.9

µH, the ripple current

will be 1750 mA under the low-input-high-output condition.

4) To meet the output noise voltage requirement, the output capacitor(s) must be chosen so that the ripple

voltage induced across the ESR of the capacitors by the output ripple current is less than 50 mV.

ESR

t 50 mV

DI

OUT

+ 42 mW

Additionally, to meet output load transient response requirements, the capacitors’ ESL and ESR must be low

enough to avoid excessive voltage transient spikes. (See Application Note U-157 for a discussion of how to

determine the amount and type of load capacitance.) For this example, four Sanyo MV-GX 1500-

µF, 6.3-V

capacitors will be used. The ESR of each capacitor is approximately 44 m

Ω so the parallel combination of four

results in an equivalent ESR of 11 m

Ω.

0.014

Ω, a gate charge requirement of 50 nC, and a turn-off time of approximately 54 ns.

To calculate the losses in the upper MOSFET, Q1, first calculate the RMS current it will be conducting.

I Q1

RMS +

d I

OUT2

)

DI

OUT2

12

Notice that with a higher output voltage, the duty cycle increases, and therefore so does the RMS current. Any

heat sink design should take into account the worst case power dissipation the device will experience.

(4)

(5)

(6)

(7)

(8)