UCC3588

5BIT PROGRAMMABLE OUTPUT BiCMOS

POWER SUPPLY CONTROLLER

SLUS311A – JULY 1999 – REVISED AUGUST 2000

8

POST OFFICE BOX 655303

APPLICATION INFORMATION

With the highest programmable output voltage of 3.5 V and the lowest possible input voltage of 4.5 V, the RMS

current Q1 will conduct is 10.5 A, and the conduction loss is:

P

CON

Q1

+ I

Q1

RMS

2

R

DS on +

1.5 W

Next, the gate drive losses are found.

P

CON

Q1

+ Q

G

f

S +

0.08 W

And the turn-off losses are estimated as

P

T OFF

Q1

+ 1

2

V

IN HI

I

D PK

tf

F

S +

0.56 W

The total loss in Q1is the sum of the three components, or about 2.1 W.

The gate drive losses in Q2 will be the same as in Q1, but the turn-off losses will be associated with the reverse

recovery of the body diode, instead of the turn-off of the channel. This is due to the UCC3588’s delay built into

the switching of the upper and lower MOSFET’s drive. For example, when Q1 is turned-off, the turn-on of Q2

is delayed for about 100 ns, insuring that the circuit has time to commutate and that current has begun to flow

in the body diode of Q2. When Q2 is turned-off, current is diverted from the channel of Q2 into the body diode

of Q2, resulting in virtually no power dissipation. When Q1 is turned ON 100ns later however, the circuit is forced

to commutate again. This time causing reverse recovery loss in the body diode of Q2 as its polarity is reversed.

The loss in the diode is expressed as:

P

RR

Q2

+ 1

2

Q

RR

V

IN HI

F

S +

0.26 W

100 ns before the turn-on of Q2, and 100 ns after the turn-off of Q2, current flows through Q2’s intrinsic body

diode. The power dissipation during this interval is:

P

COM

Q2

DIODE +

I

OUT

V

DIODE

200 ns

3.33

ms +

12

1.4

0.06

+ 1W

in Q1 was at the low-input and high-output condition, the highest RMS current in Q2 is found when the input

is at its highest, and the output is at its lowest. The equation for the RMS current in Q2 is:

I Q2

RMS

1

* d

MIN *

200 ns

3.33

ms

I

OUT

2 ) D

I

OUT

2

12

+

+ 8.7 A

P

CON

Q2

+ 1 Q2

RMS

2

R

DS on +

1.06 W

The worst case loss in Q2 comes to about 2.4 W.

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