ISL78214_15 Datasheet, PDF (12/15 Pages) Intersil Corporation – 4A Low Quiescent Current High Efficiency Synchronous Buck Regulator

ISL78214
with the 27pF and 390kΩ RC network. The maximum EAMP
voltage output is precisely clamped to 1.6V.
V_EAMP
V_CSA
DUTY
CYCLE
I_L
V_OUT
FIGURE 36. PWM OPERATION WAVEFORMS
SKIP Mode
Pulling the SYNCH pin LO (<0.4V) forces the converter into PFM
mode. The ISL78214 enters a pulse-skipping mode at light load
to minimize the switching loss by reducing the switching
frequency. Figure 37 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
and 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.
PWM
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 200mV/A, from the P-MOSFET current to the
CSA output. When the CSA output reaches 1.4V, which is equivalent
to 5.7A 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.
PG
During power-up, the open-drain power-good output holds low for
about 1ms after V_OUT reaches the regulation voltage. The PG
output also serves as a 1ms delayed power-good signal when the
pull-up resistor R₁ is installed.
PFM
CLOCK
I_L
0
V_OUT
8 CYCLES
PFM CURRENT LIMIT
NOMINAL +1.5%
NOMINAL
FIGURE 37. SKIP MODE OPERATION WAVEFORMS
Submit Document Feedback 12
LOAD CURRENT
FN7551.3
May 12, 2015