LT1107 Linear Technology, LT1107 Datasheet - Page 11

• Manufacturer Part Number: LT1107
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

A
Figure 7 details current limit circuitry. Sense transistor A1,
whose base and emitter are paralleled with power switch
Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current is
passed through internal 80 resistor R1 and out through
the I
between I
cient switch current flows to develop a V
R
turning off the switch. Delay through this circuitry is
approximately 800ns. The current trip point becomes less
accurate for switch ON times less than 3 s. Resistor
values programming switch ON time for 800ns or less will
cause spurious response in the switch circuitry although
the device will still maintain output regulation.
Using the Gain Block
The gain block (GB) on the LT1107 can be used as an error
amplifier, low-battery detector or linear post regulator.
The gain block itself is a very simple PNP input op amp with
an open collector NPN output. The negative input of the
gain block is tied internally to the 1.25V reference. The
positive input comes out on the SET pin.
Arrangement of the gain block as a low-battery detector is
straightforward. Figure 8 shows hookup. R1 and R2 need
only be low enough in value so that the bias current of the
SET input does not cause large errors. 33k for R2 is
adequate. R3 can be added to introduce a small amount of
hysteresis. This will cause the gain block to “snap” when
the trip point is reached. Values in the 1M to 10M range are
optimal. The addition of R3 will change the trip point,
however.
Output ripple of the LT1107, normally 50mV at 5V
be reduced significantly by placing the gain block in front
of the FB input as shown in Figure 9. This effectively
reduces the comparator hysteresis by the gain of the gain
block. Output ripple can be reduced to just a few millivolts
using this technique. Ripple reduction works with step-
down or inverting modes as well. For this technique to be
effective, output capacitor C1 must be large, so that each
switching cycle increases V
1000 F is a good starting value. C1 should be a low ESR
type as well.
LIM
PPLICATI
LIM
, Q3 turns on and injects current into the oscillator,
pin. The value of the external resistor connected
LIM
and V
O
U
IN
S
sets the current limit. When suffi-
I FOR ATIO
U
OUT
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
by only a few millivolts.
W
BE
across R1 +
U
OUT
can
V
BAT
V
BAT
V
OUT
Figure 9. Output Ripple Reduction Using Gain Block
Figure 8. Setting Low-Battery Detector Trip Point
=
R3
270k
R2
R1
Figure 7. LT1107 Current Limit Circuitry
V
OSCILLATOR
+ 1 1.25V
IN
R1
R2
Q3
AO
FB
GND
I
LIM
LT1107
1.25V
SET
REF
(EXTERNAL)
DRIVER
SW2
V
R
IN
LIM
SW1
SET
+
–
GND
L1
V
IN
I
LIM
Q1
R1
80
(INTERNAL)
LT1107
R3
AO
R1 =
V
R2 = 33k
R3 = 1.6M
D1
LB
SW1
= BATTERY TRIP POINT
Q2
V
LB
1107 F07
5V
35.1 A
SW2
R2
R1
– 1.25V
47k
LT1107
+
TO
PROCESSOR
11
1107 F09
C1
1107 F08
V
OUT