LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 4

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

FU CTIO AL DIAGRA
OPERATIO
LTC1779
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
output of the error amplifier EAMP. An external resistive
divider connected between V
EAMP to receive an output feedback voltage V
4
U
0.3V
V
IN
GND
2
U
U
(Refer to Functional Diagram)
FOLDBACK
FREQ
OSC
OUT
+
SHORT-CIRCUIT
–
–
+
TH
DETECT
ICMP
and ground allows the
/RUN pin, which is the
W
0.5 A
UNDERVOLTAGE
SLOPE
COMP
REFERENCE
VOLTAGE
LOCKOUT
V
+
–
IN
FB
. When the
0.15V
0.3V
V
0.8V
REF
0.325V
load current increases, it causes a slight decrease in V
relative to the 0.8V reference, which in turn causes the
I
current matches the new load current.
The main control loop is shut down by pulling the I
pin low. Releasing I
current source to charge up the external compensation
network. When the I
control loop is enabled with the I
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
TH
–
+
1
/RUN voltage to increase until the average inductor
I
RS1
R
S
TH
/RUN
+
–
Q
SHDN
BURST
CMP
CMP
SLEEP
SWITCHING
OVERTEMP
LOGIC AND
BLANKING
CIRCUIT
DETECT
SHDN
UV
TH
TH
/RUN pin reaches 325mV, the main
/RUN allows an internal 0.5 A
EAMP
OVP
V
IN
+
–
+
–
TH
1.2V
/RUN voltage then
V
+
60mV
V
0.8V
REF
REF
SENSE
V
4
4
IN
1
2
1779FD
–
V
3
FB
V
TH
5
IN
24
/RUN
SW
6
FB