ISL78113A
FN8638 Rev 0.00 Page 5 of 12
May 1, 2014
Detailed Description
Current Mode PWM Operation
The control scheme of the device is based on the peak current
mode control and the control loop is compensated internally. The
peak current of the N-channel MOSFET switch is sensed to limit
the maximum current flowing through the switch and the
inductor. The typical current limit is set to 1.3A.
The control circuit includes a current ramp generator, slope
compensator, error amplifier and a PWM comparator (see the
“Block Diagram” on page 2). The ramp signal is derived from the
inductor current. This ramp signal is then compared to the error
amplifier output to generate the PWM gating signals for driving
both N-channel and P-channel MOSFETs. The PWM operation is
initialized by the clock from the internal oscillator (typical 2MHz).
The N-channel MOSFET is turned ON at the beginning of a PWM
cycle, the P-channel MOSFET remains OFF, and the current starts
ramping up. When the sum of the ramp and the slope
compensator output reaches the error amplifier output voltage,
the PWM comparator outputs a signal to turn OFF the N-channel
MOSFET. Here, both MOSFETs remain OFF during the dead-time
interval. Next, the P-channel MOSFET is turned ON and remains
ON until the end of this PWM cycle. During this time, the inductor
current ramps down until the next clock. At this point, following a
short dead time, the N-channel MOSFET is again turned ON,
repeating as previously described.
Synchronous Rectifier
The ISL78113A integrates one N-channel MOSFET and one
P-channel MOSFET to realize a synchronous boost converter.
Because the commonly used discrete Schottky rectifier is
replaced with the low rDS(ON) P-channel MOSFET, the power
conversion efficiency reaches a value above 90%.
VOUT Isolation
Since a typical step-up converter has a conduction path from the
input to the output via the body diode of the P-channel MOSFET,
a special circuit (see the “Block Diagram” on page 2) is used to
reverse the polarity of the P-channel body diode when the device
is shut down. Thus, this configuration completely disconnects the
load from the input during shutdown of the converter. The benefit
of this feature is that the battery will not be completely depleted
during shutdown of the converter. No additional components are
needed to disconnect the battery from the output of the
converter.
Soft-Start
The soft start-up duration is the time between the device being
enabled and VOUT rising to within 3% of the target voltage. When
the device is enabled, the start-up cycle starts with a linear
operating phase. During the linear phase, the rectifying switch is
turned ON in a current limited configuration, delivering about
350mA, until the output capacitor is charged to approximately
90% of the input voltage. At this point, PWM operation begins in
boost mode. If the output voltage is below 2.3V, PWM switching
is done at a fixed duty-cycle of 75% until the output voltage
reaches 2.3V. When the output voltage exceeds 2.3V, the
closed-loop current mode PWM loop overrides the duty cycle until
the output voltage is regulated. Peak inductor current is ramped
to the current limit value (typically 1.3A) during the soft-start
period to limit in-rush current from the input source. Fault
monitoring begins approximately 2ms after the device is
enabled.
To start up with a slow VBAT ramp-up rate is likely to cause the
device to enter hiccup mode. This is a result of the input voltage
dropping due to start-up current, which causes a fault of VOUT out
of regulation, especially at high load and cold temperature.
Check the input ramp-up rate and a faster input slew rate would
help to resolve this.
Over-temperature Protection (OTP)
The device offers over-temperature protection. A temperature
sensor circuit is integrated and monitors the internal IC
temperature. Once the temperature exceeds the preset threshold
(typically +150°C), the IC shuts down immediately. The OTP has
a typical hysteresis of +25°C. When the device temperature
decreases by this, the device starts operating.