Si9142

Vishay Siliconix

S-60752—Rev. C, 05-Apr-99

FaxBack 408-970-5600, request 70750

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capacitance, and should not be used for any other external

Pin 17. LX - Inductor Node

The LX node is used internally to float the high-side n-channel

MOSFET gate drive. During the on-time of this MOSFET, the

is also used internally as the negative sense voltage for over-

current protection.

Pin 19. BST - Bootstrap Voltage

The external high-side n-channel MOSFET gate drive voltage

input

supply

voltage.

The

external

100-nF

capacitor

when the low-side MOSFETs are turned off, BST is internally

is turned on at startup to ensure initial charging of the BST

capacitor.

Pin 20. ICS - Programmable Over-Current Protection

The over-current protection circuit senses the voltage across

the external high-side n-channel MOSFET to determine the

presence of an over-current condition. Current sensing occurs

only during the on-time of this MOSFET. The trigger level of

the over-current circuit is programmable by selecting the

over-current circuit has been triggered, it disables both output

gate drives within 250 nsec. The circuit also discharges the

soft-start capacitor as shown in the timing diagram in

Figure 3.

Under Voltage Lock-Out (UVLO)

The internal UVLO circuit is designed to prevent a converter

from starting when insufficient input voltage is present. UVLO

disables the oscillator, soft-start and output drives of the

circuit has 200-mV hysteresis to prevent turn-on and -off

oscillations. When the oscillator is disabled, the Si9142 is in

stand-by mode, and consumes only 150 µA of supply current.

Start-up Timing Sequence

stabilized and is regulating. The UVLO circuit enables the

oscillator and the soft-start circuits. Once the soft-start voltage

exceeds 1.5 V, the gate drive pulses begin, with the duty cycle

of the high-side MOSFET beginning at 0% and gradually

increasing until the output voltage is in regulation.

APPLICATIONS

Setting the Current Limit

The current limit is set by comparing the voltage drop across

the external high-side n-channel MOSFET with the voltage

equation governing the overcurrent threshold is:

170

µA * R = I

Once the on-state resistance of the MOSFET is known, R can

be selected to set the desired current limit. One caution is in

order: since the MOSFET will normally be quite warm, the

resistance used in the equation should be the maximum

resistance at elevated temperatures, not typical resistance at

25

°C. The designer should also leave adequate margin above

the normal output current, both to account for tolerances and

noise in the IC, as well as to permit any initial high currents

while charging output capacitors.

The Boost Diode

The application circuit shows the use of a 1N4148 diode for

the boost circuit. This provides a low-cost component for this

application. However, it may be advantageous in some circuits

to use a Schottky diode instead. The difference is that the

Schottky has less forward drop than the regular rectifier, and

this in turn means a somewhat greater gate drive voltage for

the external high-side MOSFET. For MOSFETs with high gate

threshold and/or low transconductance, the additional gate

drive may prove very beneficial in terms of the heating of the

MOSFET, and in turn the efficiency of the converter. A ½-A,

30-V Schottky works well in this application.

Grounding

The Si9142 is provided with both analog and power ground

pins (AGND and PGND, respectively). Because of the high

gate drive currents the Si9142 can source, it is essential that

these two grounds be separated. PGND should be attached

to the source of the external low-side MOSFET; AGND should

be attached to the small-signal components of the circuit,

such as the timing resistor and the feedback resistor. Each of

these grounds should be run back independently to the input

line capacitors, to avoid ground loops.