Document Number: 70845

S-72058-Rev. D, 08-Oct-07

www.vishay.com

7

Vishay Siliconix

Si9165

DETAIL OPERATIONAL DESCRIPTION (CONT’D)

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.