ISL8013A

FN7526 Rev 2.01

Page 13 of 17

Dec 3, 2021

SKIP Mode

Pulling the SYNCH pin LO (<0.4V) forces the converter into PFM

mode. The ISL8013A enters a pulse-skipping mode at light load

to minimize the switching loss by reducing the switching

frequency. Figure 36 illustrates the skip-mode operation. A

zero-cross sensing circuit shown in Figure 2 monitors the

N-MOSFET current for zero crossing. When 8 consecutive cycles

of the inductor current crossing zero are detected, the regulator

enters the skip mode. During the eight detecting cycles, the

current in the inductor is allowed to become negative. The

counter is reset to zero when the current in any cycle does not

cross zero.

Once the skip mode is entered, the pulse modulation starts being

controlled by the SKIP comparator shown in Figure 2. Each pulse

cycle is still synchronized by the PWM clock. The P-MOSFET is

turned on at the clock's rising edge and turned off when the

output is higher than 1.5% of the nominal regulation or when its

current reaches the peak skip current limit value. Then the

inductor current is discharging to 0A and stays at zero. The

internal clock is disabled. 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-MOSFET

will be turned on again at the rising edge of the internal clock as

it repeats the previous operations.

The regulator resumes normal PWM mode operation when the

output voltage drops 1.5% below the nominal voltage.

Synchronization Control

The frequency of operation can be synchronized up to 4MHz by an

external signal applied to the SYNCH pin. The falling edge on the

SYNCH triggers the rising edge of the LX pulse. Make sure that the

minimum on time of the LX node is greater than 140ns.

Overcurrent Protection

The overcurrent protection is realized by monitoring the CSA

output with the OCP comparator, as shown in Figure 2. The current

sensing circuit has a gain of 250mV/A, from the P-MOSFET current

to the CSA output. When the CSA output reaches 1.4V, which is

equivalent to 4.8A for the switch current, the OCP comparator is

tripped to turn off the P-MOSFET immediately. The overcurrent

function protects the switching converter from a shorted output by

monitoring the current flowing through the upper MOSFET.

Upon detection of an overcurrent condition, the upper MOSFET

will be immediately turned off and will not be turned on again

until the next switching cycle. Upon detection of the initial

overcurrent condition, the overcurrent fault counter is set to 1. If,

on the subsequent cycle, another overcurrent condition is

detected, the OC fault counter will be incremented. If there are

17 sequential OC fault detections, the regulator will be shut down

under an overcurrent fault condition. An overcurrent fault

condition will result in the regulator attempting to restart in a

hiccup mode within the delay of four soft-start periods. At the

end of the fourth soft-start wait period, the fault counters are

reset and soft-start is attempted again. If the overcurrent

condition goes away during the delay of four soft-start periods,

the output will resume back into regulation point after hiccup

mode expires.

Short-Circuit Protection

The short-circuit protection SCP comparator monitors the VFB pin

voltage for output short-circuit protection. When the VFB is lower

than 0.2V, the SCP comparator forces the PWM oscillator

frequency to drop to 1/3 of the normal operation value. This

comparator is effective during start-up or an output short-circuit

event.

.

FIGURE 35. PWM OPERATION WAVEFORMS

VEAMP

VCSA

DUTY

CYCLE

IL

VOUT

FIGURE 36. SKIP MODE OPERATION WAVEFORMS

CLOCK

IL

VOUT

NOMINAL +1.5%

NOMINAL

PFM CURRENT LIMIT

LOAD CURRENT

0

PWM

PFM

8 CYCLES