ISL8013A
FN7526 Rev 2.00 Page 12 of 16
November 17, 2014
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