HIP6007CB datasheet(5/10 Pages) RENESAS

5 / 10 page

HIP6007

FN4307 Rev.1.00

Page 5 of 10

September 1997

Functional Description

Initialization

The HIP6007 automatically initializes upon receipt of power.

Special sequencing of the input supplies is not necessary. The

Power-On Reset (POR) function continually monitors the input

supply voltages and the enable (EN) pin. The POR monitors

the bias voltage 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). With the EN

pin held to VCC, 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.

The Power-On Reset (POR) function inhibits operation with the

chip disabled (EN pin low). With both input supplies above their

POR thresholds, transitioning the EN pin high initiates a soft

start interval.

Soft Start

The POR function initiates the soft start sequence. An internal

A current source charges an external capacitor (CSS) on the

SS pin to 4V. Soft start clamps the error amplifier output (COMP

pin) and reference input (+ terminal of error amp) to the SS pin

voltage. Figure 3 shows the soft start interval with CSS = 0.1F.

Initially the clamp on the error amplifier (COMP pin) controls the

converter’s output voltage. At t1 in Figure 3, the SS voltage

reaches the valley of the oscillator’s triangle wave. The

oscillator’s triangular waveform 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 3. At t3

the SS voltage exceeds the reference voltage and the output

voltage is in regulation. This method provides a rapid and

controlled output voltage rise.

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 200

A (typical)

current sink develops a voltage across ROCSET that is

reference 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 10A current sink and

inhibits PWM operation. The soft-start function recharges CSS,

and PWM operation resumes with the error amplifier clamped

to the SS voltage. Should an overload occur while recharging

CSS, the soft start function inhibits PWM operation while fully

charging CSS to 4V to complete its cycle. Figure 4 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

conditions of Figure 4 is 2.5W.

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

(IPEAK) determined by:

where IOCSET is the internal OCSET current source (200A -

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:

1. The maximum rDS(ON) at the highest junction temperature.

FIGURE 3. SOFT-START INTERVAL

TIME (5ms/DIV)

SOFT-START

(1V/DIV)

OUTPUT

(1V/DIV)

VOLTAGE

FIGURE 4. OVER-CURRENT OPERATION

TIME (20ms/DIV)

10A

15A

IPEAK

IOCSET ROCSET



rDS ON



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