UCC2585

UCC3585

SLUS304F − JULY 1999 − REVISED JANUARY 2005

7

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APPLICATION INFORMATION

soft-start

The SS pin provides a way to prevent overshoot of the output voltage by slowly increasing the duty cycle of the

PDRV output. A capacitor on SS to ground provides a controlled start-up of the supply. During start-up the

COMP pin is directly clamped to the SS pin. The SS pin has an internal current source of 13.5

µA (typical) which

charges the SS capacitor. Figure 1 shows the waveforms during softstart. The SS pin charges the external

capacitor to VIN volts after start-up is complete.

SS

COMP

VOUT

0.4 V

VIN

ENB or VIN

switching

disabled until

SS reaches

0.4 V

switching

starts, V OUT

charging up

VOUT in

regulation, SS

continues to

charge to VIN

VOUT in

regulation, SS

charged to VIN

t SS

V = 0.4 V + D x 2.1

t

Figure 1. Waveforms During Softstart

The softstart time is approximately:

t

SS +

C

SS

0.4

)

V

OUT

V

IN

2.1

13.5

mA

current limit operation

The UCC3585 has a user configurable current limit for output overload protection. To reduce external

The ISENSE pin is connected to the P-channel MOSFET drain, which is internally connected to the negative

input to the current-sense comparator. The positive comparator input is connected to the CLSET pin, which has

an internal current sink of 11.5

µA (typical). For highest accuracy, this current sink is derived from the ISET

circuitry. A resistor from VIN to CLSET sets the current limit threshold. To eliminate errors due to PCB trace

impedances, the CLSET resistor should be connected directly to the P-channel MOSFET source, and the

ISENSE pin should be directly connected to the P-channel MOSFET drain. Figure 2 shows a simplified diagram

of the current limit circuitry.

UCC3585 Not Recommended for New Designs