ISL9305H

7

FN7724.1

October 5, 2011

Theory of Operation

DCD1 and DCD2 Introduction

Both the DCD1 and DCD2 converters on ISL9305H use the

peak-current-mode pulse-width modulation (PWM) control scheme

for fast transient response and pulse-by-pulse current limiting.

Both converters are able to supply up to 1.5A load current. The

default output voltage ranges from 0.8V to 3.6V depending on the

interface in the range of 0.825V to 3.6V at 25mV/step with a

programmable slew rate. An open-drain DCDPG (DCD Power-Good)

signal is also provided to monitor the DCD1 and DCD2 output

voltages. Optionally, both DCD1 and DCD2 can be programmed to

be actively discharged via an on-chip bleeding resistor (typical

115Ω) when the converter is disabled.

Skip Mode (PFM Mode) for DCD1/DCD2

Under light load condition, the DCD1 and DCD2 can be programmed

to automatically enter a pulse-skipping mode to minimize the

switching loss by reducing the switching frequency. Figure 3

illustrates the skip mode operation. A zero-cross sensing circuit

monitors the current flowing through SW node for zero crossing.

When it is detected to cross zero for 16 consecutive cycles, the

regulator enters the skip mode. During the 16 consecutive cycles,

the inductor current could be negative. The counter is reset to zero

when the sensed current flowing through SW node does not cross

zero during any cycle within the 16 consecutive cycles. Once the

converter enters the skip mode, the pulse modulation is controlled

by an internal comparator while each pulse cycle remains

synchronized to the PWM clock. The P-Channel MOSFET is turned on

at the rising edge of the clock and turned off when its current

reaches ~20% of the peak current limit. As the average inductor

current in each cycle is higher than the average current of the load,

the output voltage rises cycle-over-cycle. When the output voltage is

sensed to reach 1.5% above its nominal voltage, the P-Channel

MOSFET is turned off immediately and the inductor current is fully

discharged to zero and stays at zero. The output voltage reduces

gradually due to the load current discharging the output capacitor.

When the output voltage drops to the nominal voltage, the P-

Channel MOSFET will be turned on again, repeating the previous

operations.

The regulator resumes normal PWM mode operation when the

output voltage is sensed to drop below 1.5% of its nominal

voltage value.

Soft-Start

The soft-start reduces the in-rush current during the start-up stage.

The soft-start block limits the current rising speed so that the

output voltage rises in a controlled fashion.

Overcurrent Protection

The overcurrent protection for DCD1 and DCD2 is provided on

ISL9305H for when an overload condition occurs. When the current

at P-Channel MOSFET is sensed to reach the current limit, the

internal protection circuit is triggered to turn off the P-Channel

MOSFET immediately.

DCD Short-Circuit Protection

The ISL9305H provides Short-Circuit Protection for both DCD1 and

DCD2. The feedback voltage is monitored for output short-circuit

protection. When the output voltage is sensed to be lower than a

certain threshold, the internal circuit will change the PWM oscillator

frequency to a lower frequencies in order to protect the IC from

damage. The P-Channel MOSFET peak current limit remains active

during this state.

FIGURE 2. PWM OPERATION WAVEFORMS

d

FIGURE 3. SKIP MODE OPERATION WAVEFORMS

16 CYCLES

CLOCK

0

20% PEAK CURRENT LIMIT