ltc1772b Linear Technology Corporation, ltc1772b Datasheet - Page 4

• Manufacturer Part Number: ltc1772b
• Description: Constant Frequency Current Mode Step-down Dc/dc Controller In Sot-23
• Manufacturer: Linear Technology Corporation

FUNCTIONAL DIAGRA
OPERATIO
LTC1772B
Main Control Loop
The LTC1772B is a constant frequency current mode
switching 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
GND
V
2
5
IN
U
SENSE
4
4
U
–
(Refer to Functional Diagram)
FOLDBACK
15mV
FREQ
OSC
+
–
SHORT-CIRCUIT
OUT
+
–
DETECT
TH
ICMP
and ground allows the
/RUN pin, which is the
W
0.5 A
UNDERVOLTAGE
SLOPE
COMP
REFERENCE
LOCKOUT
VOLTAGE
V
–
+
IN
FB
. When the
0.3V
V
0.8V
REF
0.35V
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 0.85V.
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
CMP
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
1.2V
V
IN
/RUN voltage then
V
+
60mV
V
0.8V
REF
REF
V
IN
1772FD
PGATE
V
6
3
FB
TH
/RUN
FB