SI9165_07 Datasheet, PDF (7/10 Pages) Vishay Siliconix – High Frequency 600-mA Synchronous Buck/Boost Converter

DETAIL OPERATIONAL DESCRIPTION (CONT’D)
Si9165
Vishay Siliconix
Over Temperature Protection
The Si9165 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 Si9165
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. 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 Si9165 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 Si9165 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 µA, instead of 500 µA 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 µH 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 Si9165 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 µF capacitor.
Document Number: 70845
S-72058-Rev. D, 08-Oct-07
www.vishay.com
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