ISL9307

7

FN7931.3

September 20, 2012

Theory of Operation

DCD1 and DCD2

Both the DCD1 and DCD2 converters on ISL9307 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 1500mA load

current.

Under light load conditions, the device enters a pulse-skipping

mode to minimize switching loss by reducing switching

frequency. Figure 2 illustrates the skip mode operation.

A zero-cross sensing circuit monitors the current flowing through

the SW node for zero crossing. When it is detected to cross zero

for 16 consecutive cycles, the regulator enters 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 the SW node does not cross zero during any cycle within

the 16 consecutive cycles.

Once the converter enters 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 turns 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, as shown in Figure 3.

16 CYCLES

CLOCK

0

20% PEAK CURRENT LIMIT

FIGURE 2. SKIP MODE OPERATION WAVEFORMS

d

FIGURE 3. PWM OPERATION WAVEFORMS