Datasheet
4
LTC1779
(Refer to Functional Diagram)
Main Control Loop
The LTC1779 is a constant frequency current mode switch-
ing regulator. During normal operation, the internal
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the 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 connected 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
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 325mV, 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
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
OVERTEMP
DETECT
–
+
–
+
–
+
0.15V
0.5µA
0.3V
SLEEP
OVP
SHORT-CIRCUIT
DETECT
BURST
CMP
SHDN
1.2V
UV
1779FD
V
REF
+
60mV
V
REF
0.8V
V
IN
RS1
VOLTAGE
REFERENCE
SLOPE
COMP
ICMP
R
S
Q
FREQ
FOLDBACK
OSC
SENSE
–
V
IN
5
2
4
–
+
4
EAMP
V
FB
–
+
3
SW
V
IN
2Ω
1× 24×
6
I
TH
/RUN
V
IN
0.325V
V
REF
0.8V
1
–
+
SHDN
CMP
0.3V
GND
–
+
UNDERVOLTAGE
LOCKOUT
V
IN
FU CTIO AL DIAGRA
U
U
W
OPERATIO
U