LTC1872
4
1872fa
For more information
www.linear.com/LTC1872
FuncTional DiagraM
operaTion
Main Control Loop
The LTC1872 is a constant frequency current mode
switching regulator. During normal operation, the external
N-channel power MOSFET is turned on each cycle by the
oscillator and turned off when the current comparator
(ICMP) resets the RS latch. The peak inductor current
at which ICMP resets the RS latch is controlled by the
voltage on the I
TH
/RUN pin, which is the output of the
error amplifier EAMP. An external resistive divider con-
nected between V
OUT
and ground allows the EAMP to
receive an output feedback voltage V
FB
. When the load
current increases, it causes a slight decrease in V
FB
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
–
+
–
+
–
+
0.15V
0.5µA
0.3V
SLEEP
OVP
BURST
CMP
SHDN
1.2V
UV
1872FD
V
REF
+
60mV
V
REF
0.8V
V
IN
RS
VOLTAGE
REFERENCE
SLOPE
COMP
ICMP
R
S
Q
FREQ
FOLDBACK
OSC
SENSE
–
V
IN
5
2
4
–
+
4
EAMP
V
FB
–
+
3
NGATE
V
IN
6
I
TH
/RUN
V
IN
0.35V
V
REF
0.8V
1
–
+
SHDN
CMP
0.3V
GND
–
+
UNDERVOLTAGE
LOCKOUT
V
IN
(Refer to Functional Diagram)
relative to the 0.8V reference, which in turn causes the
I
TH
/RUN voltage to increase until the average inductor
current matches the new load current.
The main control loop is shut down by pulling the I
TH
/
RUN pin low. Releasing I
TH
/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the I
TH
/RUN pin reaches 0.35V, the main
control loop is enabled with the I
TH
/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
up, the corresponding output current trip level follows,
allowing normal operation.
