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-

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-

by the resistor divider

V X

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

V X

The typical short circuit frequency is set by the following

equations

1(20 pF

kHz mA V X

where

For example say 140 kX and 100 kX were selected for

the output voltage level where the frequency starts to decay

is

kX)

1(20 pF

then becomes

kHz X

v

X

kHz

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

third the operating frequency or 188 kHz leads to a resistor

value of

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

9