Datasheet

LTC1700
6
1700fa
Main Control Loop
The LTC1700 is a constant frequency, current mode
controller for DC/DC step-up converters. In normal opera-
tion, the main external N-channel power MOSFET is turned
on when the oscillator sets a latch and turned off either
when the V
DS
sense amplifier (V
DS
) resets the latch or the
duty cycle has reached 90%. When the main MOSFET is
turned off, the synchronous rectifier P-channel MOSFET is
FUNCTIONAL DIAGRA
UU
W
turned on until either the inductor current is about to
reverse, as determined by the current reversal comparator
(I
RCMP
), or the next cycle begins. Inductor current is
measured by sensing the V
DS
potential across the con-
ducting MOSFET. The peak inductor current is controlled
by the voltage on the I
TH
pin, which is the output of the
error amplifier (EA). An external resistive divider con-
nected between V
OUT
and GND allows EA to receive an
OPERATIO
U
(Refer to Functional Diagram)
+
–
BURST
INHIBIT
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
START-UP
OSCILLATOR
7
3
2
5
+
–
SYNC/
MODE
X
Y
V
FB
1
SGND
I
TH
1.205V
1.205V
REFERENCE
8
10
9
6
90% DUTY
CYCLE LIMIT
OV
+
–
+
–
+
–
V
DS
I
RCMP
0.36V
SW
BG
TG
V
OUT
V
OUT
V
OUT
V
FB
V
REF
+ 58mV
PGND
1700 • FD
Y = “0” ONLY WHEN X IS A CONSTANT “1”, OTHERWISE Y = “1”
EA
+
–
+
–
BURST
+
–
RUN/
SOFT-START
S
QR
QB
L1
MAIN
OSC
4
SHDN
RUN/SS
V
OUT
V
OUT
SLOPE
COMP
M1
1Ω
9mV
60mV
I
CMP
= “1” WHEN V
OUT
< 2.3V
3.8µA
V
CC
0.12V
SLEEP
g
m
= 0.9m
Ω
SC