RC5054 データシートの表示（PDF） - Fairchild Semiconductor

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RC5054

Programmable Synchronous DC-DC Converter Controller for Low Voltage Microprocessors

Fairchild Semiconductor 

PRODUCT SPECIFICATION

RC5054A

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

volt-age at the VCC pin and the input voltage (VIN) on the

OCSET pin. The level on OCSET is equal to VIN less a fixed

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

+12V power source, VIN and VCC are equivalent and the

+12V power source must exceed the rising VCC threshold

before POR initiates operation.

Soft Start

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

nal 10µA current source charges an external capacitor (CSS)

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

interval with CSS = 0.1µF. Initially the clamp on the error

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

age. At t1 in Figure 1, the SS voltage reaches the valley of the

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

continues to t2. With sufficient output voltage, the clamp on

the reference input controls the output voltage. This is the

interval between t2 and t3 in Figure 1. At t3 the SS voltage

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.

PGOOD

(2V/DIV)

0V

Over-Current Protection

The over-current function protects the converter from a

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

RDS(ON) to monitor the current. This method enhances the

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

resistor (ROCSET) programs the over-current trip level. An

internal 200µA current sink develops a voltage across ROCSET

that is referenced to VIN. When the voltage across the upper

MOSFET (also referenced to VIN) exceeds the voltage across

ROCSET , the over-current function initiates a soft-start

sequence. The soft-start function discharges CSS with a 10µA

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

tion recharges CSS, and PWM operation resumes with the

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

load occur while recharging CSS, the soft start function

inhibits PWM operation while fully charging CSS to 4V to

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

(IPEAK) determined by:

IPEAK

=

I---O---C----S---E---T----•-----R----O----C---S---E---T--

RDS(ON)

4V

2V

0V

15A

10A

5A

0A

Time (20ms/DIV)

SOFT-START

(1V/DIV)

OUTPUT

VOLTAGE

(1V/DIV)

0V

0V

t1

t2

t3

Time (5ms/DIV)

Figure 1. Soft Start Interval

Figure 2. Over-Current Operation

where IOCSET is the internal OCSET current source (200µA

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

RDS(ON) variations. To avoid over-current tripping in the

normal operating load range, find the ROCSET resistor from

the equation above with:

• The maximum RDS(ON) at the highest junction temperature.

• The minimum IOCSET from the specification table.

• Determine IPEAK for IPEAK > IOUT(MAX) + (∆I)/2,

where ∆I is the output inductor ripple current.

5

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