HR1000A – RESONANT HALF-BRIDGE CONTROLLER
PIN FUNCTIONS
Pin #
Name Description
Soft-start. Connect an external capacitor with this pin to GND and a resistor to Fset pin to
set the maximum oscillator frequency and the time constant for the frequency shift during
start-up. An internal switch discharges the capacitor when the chip turns off (VCC < UVLO,
VCC > 16V, BO < 1.25V or > 5.5V, LATCH > 1.85V, CS >1.5V, TIMER > 2V, thermal
shutdown) to guarantee soft-start when the current sense pin voltage exceeds 0.8V, and as
long as it stays above 0.75V.
1
SS
Period between over-current and shutdown. Connect a capacitor and a resistor from this
pin to GND to set both the maximum duration from an over-current condition before the IC
stops switching, and the delay before the IC resumes switching. Each time the CS pin
voltage exceeds 0.8V, an internal 130µA source charges the capacitor; an external resistor
slowly discharges this capacitor. If the pin voltage reaches 2V, the soft-start capacitor
discharges completely, pushing the switching frequency to its maximum value; the 130µA
source remains on. When the voltage exceeds 3.5V the IC stops switching and the internal
current source turns off so that the pin voltage decays. The IC enters soft-started when the
voltage drops below 0.3V. This allows the converter to work intermittently with very low
average input power under short-circuit conditions.
2
TIMER
Time-Set. An internal current source programmed by the external network connected to pin
4 charges and discharges a capacitor connected to GND. Determines the converter’s
switching frequency.
Switching Frequency Set. Provides a precise 2V reference. A resistor connected from this
pin to GND defines a current that sets the minimum oscillator frequency. Connect the
phototransistor of an opto-coupler to this pin through a resistor to close the feedback loop
that modulates the oscillator frequency to regulate the converter output voltage. The value
of this resistor will set the maximum operating frequency. An R-C series connected from
this pin to GND sets frequency shift at start-up to prevent excessive energy inrush (soft-
start).
3
4
CT
Fset
Burst-Mode Operation Threshold. The pin senses some voltage related to the feedback
control, which is compared to an internal reference (1.25V). If the voltage on the pin is
lower than the reference, the IC enters an idle state and reduces its quiescent current. The
chip resumes switching when the voltage exceeds the reference by 50mV. Soft-start is not
invoked. This function enables burst-mode operation when the load falls below a
programmed level, determined by connecting an appropriate resistor to the opto-coupler to
pin Fset (see block diagram). Tie the pin to Fset if burst-mode is not used.
5
Burst
Primary-Current Sense. Uses a sense resistor or a capacitive divider to sense the primary
current. The voltage signal requires an averaging filter because this input is not intended
for cycle-by-cycle control. As the voltage exceeds a 0.8V threshold (with 50mV hysteresis),
the soft-start capacitor on pin 1 discharges internally: The frequency increases, limiting the
power throughput. Under an output short circuit, this normally results in a nearly-constant
peak-primary current. A timer set on pin 2 limits the duration of this condition. If the current
continues to build up despite the frequency increase, a second comparator referenced at
1.5V latches the device off and brings its consumption up to about pre-start-up levels. The
information is latched, requiring cycling the IC supply voltage to restart: The latch is
removed as the VCC voltage drops below the UVLO threshold. Tie the pin to GND if the
function is not used.
6
CS
HR1000A Rev. 1.01
8/30/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
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