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AN701 Vishay Siliconix Document Number: 70575 16-Jan-01 1 Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller Vishay Siliconix's Si9100 monolithic switchmode PWM controller established the standard for low-power, high- efficiency dc-to-dc converters. This versatile device integrates a number of useful features, including a power MOSFET, high-voltage start circuitry, and low power consumption. The Si9114A controller pushes the limits for high-frequency power conversion by further reducing delay times and adding additional features. As a result, dc-to-dc converters can be designed for frequencies up to 1 MHz with simple PWM topologies instead of the complex resonant ones. The Si9114A uses constant frequency current mode control. By increasing the conversion frequency, power supply designers will be able to: reduce the size of energy storage components increase reliability by using ceramic capacitors produce an all-surface-mount assembly solution lower system costs simplify the implementation of a distributed power architecture. Traditionally, distributed power has been regarded as costly and impractical to implement due to the high cost of local power conversion and the perceived complexity of resonant power supplies. But these decision criteria are now being changed by the availability of small-outline control ICs, LITTLE FOOT MOSFETs, ceramic capacitors, and new magnetic components.

  • constant frequency current

  • input capacitor

  • up problem

  • voltage tolerance

  • designing high-frequency

  • start circuit


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AN701
Vishay Siliconix
Designing High-Frequency DC-to-DC Converters
With the Si9114A Switchmode Controller

Vishay Siliconix’s Si9100 monolithic switchmode PWM reduce the size of energy storage components
controller established the standard for low-power, high- increase reliability by using ceramic capacitors
efficiency dc-to-dc converters. This versatile device integrates
produce an all-surface-mount assembly solutiona number of useful features, including a power MOSFET,
lower system costshigh-voltage start circuitry, and low power consumption. The
Si9114A controller pushes the limits for high-frequency power simplify the implementation of a distributed power
conversion by further reducing delay times and adding architecture.
additional features. As a result, dc-to-dc converters can be
designed for frequencies up to 1 MHz with simple PWM Traditionally, distributed power has been regarded as costly
topologies instead of the complex resonant ones. and impractical to implement due to the high cost of local
power conversion and the perceived complexity of resonant
power supplies. But these decision criteria are now being
The Si9114A uses constant frequency current mode control. changed by the availability of small-outline control ICs,
By increasing the conversion frequency, power supply LITTLE FOOT MOSFETs, ceramic capacitors, and new
designers will be able to: magnetic components.
R CSYNC OSC OSCFB COMP CURRENT SENSE
5 6 13 10 8 9
Error
Amplifier
B 2 OSC
OSC
– Out4
Enable+NI
2 V
–3 +–Ref RVREF +
Gen Q
S
4.6 V
+ 12 DRIVER
20 A –
OUTPUT
7
11SS 1.25 V
–VIN
VCCSubstrate–VIN
20 A
214
SHUTDOWNV SCC
Q
R
1
+VIN –
+

Undervoltage
0.2 V– Lockout+
+ 9.2 V
Figure 1 Si9114A Block Diagram
Document Number: 70575 www.vishay.com
16-Jan-01 1AN701
Vishay Siliconix
depletion mode MOSFET. The 9.2-V threshold has a

hysteresis of 300 mV to prevent oscillations when the

transition voltage is not clearly defined or when high-line
supply impedance is encountered.
The Si9114A controller is similar in configuration to the Si9110.
It uses a traditional constant frequency current mode control,
For applications where the input dc voltage is not high, and thethe most commonly used architecture. The duty cycle is limited
chip power consumption is not excessive, the feedbackto less than 50% to avoid problems with core reset.
winding can be eliminated. In such cases, the pre-regulator
circuit will behave just like a linear regulator with 9.2-V output
Current mode control is presently the de-facto standard for
and 10-k series resistance. In this case, the parameters to be
PWM control circuits. Indeed, it is the only candidate that
considered are the dropout voltage at lowest line condition and
should be considered, given its many advantages:
the power dissipation at highest voltage. The high-voltage
Cycle by cycle current limit protection depletion mode MOSFET contains an internal body diode, and
in situations where the V is being powered from a laboratoryCC Simple loop compensation, eliminating effect of output
supply, care must be taken to avoid loading the +V railINinductor
beyond the current rating of this device. Typically, the reverse
Excellent fast transient response due to inner control loop characteristics of the device will generate a voltage of 3.4 V on
Automatic input voltage feed-forward compensation Pin 1 with 10-k load when powering V from a lab supply.CC
In some applications it is necessary to inhibit the start of a