SI9169 Datasheet, PDF (7/8 Pages) Vishay Siliconix – High Frequency 1-A Synchronous Buck/Boost Converter

Si9169
Vishay Siliconix
Over Temperature Protection
The Si9169 is designed with over temperature protection
circuit to prevent MOSFET switches from running away. If the
temperature reaches 165_C, internal soft-start capacitor is
discharged, shutting down the output stage. Converter
remains in the disabled mode until the temperature in the IC
decreases below 140_C.
PWM Mode
With PWM/PSM mode pin in logic high condition, the Si9169
operates in constant frequency (PWM) mode. As the load and
line varies, switching frequency remain constant. The
switching frequency is programmed by the Rosc value as
shown by the Oscillator curve. In the PWM mode, the
synchronous drive is always enabled, even when the output
current reaches 0 A to assure the converter is operating in
continuous current mode. In continuous current mode,
transfer function of the converter remain constant, providing
fast transient response. If the converter operates in
discontinuous current mode, overall loop gain decreases and
transient response time can be ten times longer than if the
converter remain in continuous current mode. This transient
response time advantage can significantly decrease the
hold-up capacitors needed on the output of dc-dc converter to
meet the transient voltage regulation. Therefore, the
PWM/PSM pin is available to dynamically program the
controller.
The maximum duty cycle of the Si9169 can reach 100% in
buck mode. This allows the system designers to extract out
the maximum stored energy from the battery. Once the
controller delivers 100% duty cycle, converter operates like a
saturated linear regulator. At 100% duty cycle, synchronous
rectification is completely turned off. Up to a maximum duty
cycle of 80% at 2-MHz switching frequency, controller
maintains perfect output voltage regulation. If the input voltage
drops below the level where the converter requires greater
than 80% duty cycle, controller will deliver 100% duty cycle.
This instantaneous jump in duty cycle is due to fixed BBM time,
MOSFET delay/rise/fall time, and the internal propagational
delays. In order to maintain regulation, controller might
fluctuate its duty cycle back and forth from 100% to something
less than maximum duty cycle while the converter is operating
in this input voltage range. If the input voltage drops further,
controller will remain on 100%. If the input voltage increases
to a point where it requires less than 80% duty cycle,
synchronous rectification is once again activated.
The maximum duty cycle under boost mode is internally limited
to 75% to prevent inductor saturation. If the converter is turned
on for 100% duty cycle, inductor never gets a chance to
discharge its energy and eventually saturates. In boost mode,
synchronous rectifier is always turned on for minimum or
greater duration as long as the switch has been turned on. The
controller will deliver 0% duty cycle, if the input voltage is
greater than the programmed output voltage. Because of
signal propagation time and MOSFET delay/rise/fall time,
controller will not transition smoothly from minimum
controllable duty cycle to 0% duty cycle. For example,
controller may decrease its duty cycle from 5% to 0% abruptly,
instead of gradual decrease you see from 75% to 5%.
Pulse Skipping Mode
The gate charge losses produced from the Miller capacitance
of MOSFETs are the dominant power dissipation parameter
during light load (i.e. < 10 mA). Therefore, less gate switching
will improve overall converter efficiency. This is exactly why
the Si9169 is designed with pulse skipping mode. If the
PWM/PSM pin is connected to logic low level, converter
operates in pulse skipping modulation (PSM) mode. During
the pulse skipping mode, quiescent current of the controller is
decreased to approximately 200 mA, instead of 500 mA during
the PWM mode. This is accomplished by turning off most of
internal control circuitry and utilizing a simple constant on-time
control with feedback comparator. The controller is designed
to have a constant on-time and a minimum off-time acting as
the feedback comparator blanking time. If the output voltage
drops below the desired level, the main switch is first turned on
and then off. If the applied on-time is insufficient to provide the
desired voltage, the controller will force another on and off
sequence, until the desired voltage is accomplished. If the
applied on-time forces the output to exceed the desired level,
as typically found in the light load condition, the converter stays
off. The excess energy is delivered to the output slowly, forcing
the converter to skip pulses as needed to maintain regulation.
The on-time and off-time are set internally based on inductor
used (1.5-mH Typical), Mode pin selection and maximum load
current. Wide duty cycle range can be achieved in both buck
and boost configurations. In pulse skipping mode,
synchronous rectifier drive is also disabled to further decrease
the gate charge loss, which in turn improves overall converter
efficiency.
Reference
The reference voltage of the Si9169 is set at 1.3 V. The
reference voltage is internally connected to the non-inverting
inputs of the error amplifier. The reference is decoupled with
0.1-mF capacitor.
Document Number: 70945
S-40695—Rev. D, 19-Apr-04
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