Si9166
Vishay Siliconix
Document Number: 70847
S-40701—Rev. C, 19-Apr-04
www.vishay.com
7
PWM Mode
With PWM/PSM mode pin in logic high condition, the Si9166
operates in constant frequency (PWM) mode. As the load and
line varies, switching frequency remain constant. The
switching frequency is programmed by the R
OSC
value. In the
PWM mode, the synchronous drive is always enabled, even
when the output current reaches 0 A. Therefore, the converter
always operates in continuous conduction mode (CCM) if a
synchronous switch is used. In CCM, transfer function of the
converter remains almost constant, providing fast transient
response. If the converter operates in discontinuous
conduction mode (DCM), 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. The PWM/PSM
pin is
available to dynamically program the controller. If the
synchronous rectifier switch is not used, the converter will
operate in DCM at light load.
The maximum duty cycle of the Si9166 can reach 100% in
buck mode. The duty cycle will continue to increase as the
input voltage decreases until it reaches 100%. This allows the
system designers to extract the maximum stored energy from
the battery. Once the controller delivers 100% duty cycle, the
converter operates like a saturated linear regulator. At 100%
duty cycle, synchronous rectification is completely turned off.
Up to 80% maximum duty cycle at 2-MHz switching frequency,
the controller maintains perfect output voltage regulation. If the
input voltage drops below the level where the converter
requires greater than 80% duty cycle, the 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 lower than 80% 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’s requiring less than 80% duty
cycle, synchronous rectification is once again activated.
The maximum duty cycle under boost mode is internally limited
to 70% 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 saturate. 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 70% 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 Si9166 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. Therefore, with this control method, duty cycle
ranging from 0 to near 100% is possible depending on whether
buck or boost is chosen. In pulse skipping mode, synchronous
rectifier drive is also disabled to further decrease the gate
charge loss and increase overall converter efficiency.
Reference
The reference voltage for the Si9166 is set at 1.3 V. The
reference voltage is internally connected to the non-inverting
inputs of the error amplifier. The reference pin requires 0.1-mF
decoupling capacitor.
Error Amplifier
The error amplifier gain-bandwidth product and slew rate are
critical parameters which determines the transient response of
converter. The transient response is function of both small and
large signal responses. The small signal response is
determined by the feedback compensation network while the
large signal is determined by the error amplifier dv/dt and the
inductor di/dt slew rate. Besides the inductance value, error
amplifier determines the converter response time. In order to
minimize the response time, the Si9166 is designed with
2-MHz error amplifier gain-bandwidth product to generate the
widest converter bandwidth and 3.5 V/msec slew rate for
ultra-fast large signal response.
