PRODUCT SPECIFICATION

RC5054A

5

Functional Description

Initialization

The RC5054A automatically initializes upon receipt of

power. Special sequencing of the input supplies is not neces-

sary. The Power-On Reset (POR) function continually moni-

tors the input supply voltages. The POR monitors the bias

voltage drop (see over-current protection). The POR function

initiates soft start operation after both input supply voltages

exceed their POR thresholds. For operation with a single

before POR initiates operation.

Soft Start

The POR function initiates the soft start sequence. An inter-

nal 10

on the SS pin to 4V. Soft start clamps the error amplifier out-

put (COMP pin) and reference input (+ terminal of error

amp) to the SS pin voltage. Figure 1 shows the soft start

amplifier (COMP pin) controls the converter’s output volt-

oscillator’s triangle wave. The oscillator’s triangular wave-

form is compared to the ramping error amplifier voltage. This

generates PHASE pulses of increasing width that charge the

output capacitor(s). This interval of increasing pulse width

the reference input controls the output voltage. This is the

exceeds the DACOUT voltage and the output voltage is in

regulation. This method provides a rapid and controlled

output voltage rise. The PGOOD signal toggles ‘high’ when

the output voltage (VSEN pin) is within

±5% of DACOUT.

The 2% hysteresis built into the power good comparators

prevents PGOOD oscillation due to nominal output voltage

ripple.

Figure 1. Soft Start Interval

0V

0V

0V

Time (5ms/DIV)

SOFT-START

(1V/DIV)

OUTPUT

(1V/DIV)

VOLTAGE

PGOOD

(2V/DIV)

Over-Current Protection

The over-current function protects the converter from a

shorted output by using the upper MOSFET’s on-resistance,

converter’s efficiency and reduces cost by eliminating a current

sensing resistor. The over-current function cycles the soft-

start function in a hiccup mode to provide fault protection. A

internal 200

current sink and inhibits PWM operation. The soft-start func-

error amplifier clamped to the SS voltage. Should an over-

complete its cycle. Figure 2 shows this operation with an

overload condition. Note that the inductor current increases

to over 15A during the CSS charging interval and causes an

over-current trip. The converter dissipates very little power

with this method. The measured input power for the condi-

tions of Figure 2 is 2.5W.

The over-current function will trip at a peak inductor current

Figure 2. Over-Current Operation

typical). The OC trip point varies mainly due to the MOSFET’s

the equation above with:

where

∆I is the output inductor ripple current.

I

PEAK

I

OCSET

R

OCSET

•

R

DS ON

()

--------------------------------------------

=

0A

0V

Time (20ms/DIV)

5A

10A

15A

2V

4V