ltc1622 Linear Technology Corporation, ltc1622 Datasheet - Page 5

• Manufacturer Part Number: ltc1622
• Description: Low Input Voltage Current Mode Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
Main Control Loop
The LTC1622 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 R
the current comparator (I
inductor current at which I
controlled by the voltage on the I
of the error amplifier EA. An external resistive divider
connected between V
an output feedback voltage V
increases, it causes a slight decrease in V
0.8V reference, which in turn causes the I
increase until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 2.5 A
FUNCTIONAL DIAGRA
U
SYNC/
MODE
V
0.8V
V
REF
FB
0.3V
V
3
IN
5
SHUTDOWN
TRIP = 1.97V
1 A
REFERENCE
U
UVLO
0.8V
V
+
–
IN
U
OUT
BURST DEFEAT
(Refer to Functional Diagram)
S
latch (R
COMP
and ground allows EA to receive
X
GND
6
COMP
RUN/SS
FB
V
) resets the latch. The peak
IN
SHIFT
FREQ
TH
OSC
. When the load current
Y
S1
pin, which is the output
resets the R
) and turned off when
W
4
V
REF
Y = “0” ONLY WHEN X IS A CONSTANT “1”
OTHERWISE Y = “1”
2.5 A
+ 0.12V
FB
V
0.8V
REF
relative to the
TH
SLOPE
COMP
voltage to
S
+
SOFT-START
–
+
–
OV
EA
latch is
g
m
RUN/
= 0.5m
–
+
current source to charge up the soft-start capacitor C
When C
with the I
maximum value. As C
ally released allowing normal operation to resume.
Comparator OV guards against transient overshoots
> 16% by turning off the P-channel power MOSFET and
keeping it off until the fault is removed.
Burst Mode Operation
The LTC1622 can be enabled to go into Burst Mode
operation at low load currents simply by leaving the SYNC/
MODE pin open or connecting it to a voltage of at least 2V.
In this mode, the peak current of the inductor is set as if
V
at the I
current is greater than the load requirement, the voltage at
ITH
I
= 0.36V (at low duty cycles) even though the voltage
TH
2
0.12V
TH
SS
TH
pin is at lower value. If the inductor’s average
reaches 0.7V, the main control loop is enabled
voltage clamped at approximately 5% of its
–
+
S
R
BURST
R
EN
S1
Q
SS
V
CC
SLEEP
continues to charge, I
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
0.36V
SENSE
1622 BD
–
1
+
ICOMP
LTC1622
8
–
V
IN
TH
V
IN
is gradu-
PDRV
7
5
SS
.