LM3101 datasheet(9/20 Pages) NSC

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Functional Description (Continued)

Frequency-Shift Circuit

The LM3101 has the ability to gradually reduce its operating

frequency during an output short circuit The amount that

the frequency shifts and the output voltage threshold deter-

mining where the frequency starts to shift are both pro-

grammed by two external resistors RSC1 and RSC2 con-

nected to the pin RSC (pin 5)

A simplified internal schematic of the Frequency Shift Circuit

is shown in

Figure 2 The oscillator operates at its nominal

frequency as long as the voltage at the emitter of the tran-

sistor Q2 is higher than the internal reference voltage VREF

Q2 emitter voltage is the output voltage VOUT scaled down

by the resistor divider

VRSC e VQ2E e VOUT RSC2 (RSC1 a RSC2)

V X

where VQ2 l VREF (124V) for normal operation

If VOUT drops due to an overload a current starts to flow

through Q2 A cascoded current mirror causes one-tenth of

this current to be subtracted from the timing capacitor

charge current Reducing the timing capacitor charge cur-

rent results in decreasing the oscillator frequency The

breakpoint where the frequency-shift starts is programmed

by the ratio of the two resistors

VOUT(SC) e 124V 1 a (RSC1 RSC2)

V X

The typical short circuit frequency is set by the following

equations

FSC e IOSC b 01 ((124V b VOUT(SC))RSC1) a

(124VRSC2)

1(20 pF

124V)

kHz mA V X

where

IOSC e 025 (124VRT)

For example say 140 kX and 100 kX were selected for

RSC1 and RSC2 respectively with RT set to 25 kX Then

the output voltage level where the frequency starts to decay

VOUT(CL) e 124V 1 a (140 kX 100 kX) j 30V

and the short circuit frequency is (IOSC e 124 mA and as-

suming VOUT is 0V during a short circuit)

FSC e 124 mA b 01 (124V140 kX) a (124V100

kX)

1(20 pF

124V) e 4143 kHz j 415 kHz

If RSC2 is omitted the frequency starts to shift when VOUT

drops below VREF The short circuit frequency equation

then becomes

FSC e (120 pF) (0267RT) b (009RSC1)

kHz X

RSC1 e 009 (0267RT) b (FSC 20 pF)

kHz

Selecting a short-circuit frequency that is greater than one-

third the operating frequency or 188 kHz leads to a resistor

value of

RSC1 e 009 (026725 kX) b (188 kHz 20 pF) e 13 kX

Mode Control

The LM3101 can operate in voltage mode current mode or

charge mode control Two multi-function pins are involved in

setting the operating mode the Mode Control pin (MC - pin

2) and the Current Mode Input pin (CMI - pin 6)

Figure 3

shows the simplified schematic diagram of the mode control

circuit To operate with voltage mode control the MC pin is

pulled high with a resistor (typically 3 kX) and the CMI pin is

connected to ground The mode comparator senses the MC

pin voltage and sets the mode control multiplexer to voltage

mode control Notice that there is a 56V zener diode clamp-

ing the MC pin voltage

TLH11436 – 8

FIGURE 3 Simplified Version

of the Mode Control Circuit

TLH11436 – 9

FIGURE 4 Current Mode Sense Circuit

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