LTC1772 Linear Technology, LTC1772 Datasheet - Page 4

• Manufacturer Part Number: LTC1772
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• Manufacturer: Linear Technology

FUNCTIONAL DIAGRA
OPERATIO
LTC1772
Main Control Loop
The LTC1772 is a constant frequency current mode switch-
ing regulator. During normal operation, the external
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
V
2
5
U
IN
SENSE
U
4
4
–
(Refer to Functional Diagram)
FOLDBACK
FREQ
OSC
OUT
+
–
TH
SHORT-CIRCUIT
and ground allows the
+
–
/RUN pin, which is the
W
DETECT
ICMP
0.5 A
UNDERVOLTAGE
SLOPE
COMP
REFERENCE
VOLTAGE
LOCKOUT
FB
V
. When the
–
+
IN
0.15V
0.3V
V
0.8V
REF
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
0.35V
/RUN voltage to increase until the average inductor
+
–
1
I
RS1
R
S
TH
/RUN
+
–
Q
SHDN
BURST
CMP
CMP
SLEEP
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
SHDN
UV
TH
TH
/RUN pin reaches 0.35V, the main
/RUN allows an internal 0.5 A
EAMP
OVP
TH
+
–
+
–
/RUN voltage then
1.2V
V
IN
V
+
60mV
V
0.8V
REF
REF
V
IN
1772FD
PGATE
V
TH
6
3
FB
/RUN
FB