Datasheet

5
LTC1622
FUNCTIONAL DIAGRA
UU
W
OPERATIO
U
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
S
latch (R
S1
) and turned off when
the current comparator (I
COMP
) resets the latch. The peak
inductor current at which I
COMP
resets the R
S
latch is
controlled by the voltage on the I
TH
pin, which is the output
of the error amplifier EA. An external resistive divider
connected between V
OUT
and ground allows EA 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
voltage to
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 2.5µA
(Refer to Functional Diagram)
current source to charge up the soft-start capacitor C
SS
.
When C
SS
reaches 0.7V, the main control loop is enabled
with the I
TH
voltage clamped at approximately 5% of its
maximum value. As C
SS
continues to charge, I
TH
is gradu-
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
ITH
= 0.36V (at low duty cycles) even though the voltage
at the I
TH
pin is at lower value. If the inductor’s average
current is greater than the load requirement, the voltage at
+
–
+
–
+
–
+
–
+
–
+
–
BURST DEFEAT
1µA
5
SYNC/
MODE
3
6
4
2
S
RQ
BURST
R
S1
0.12V
SLEEP
SENSE
–
EN
1
0.36V
8
PDRV
OV
1622 BD
7
GND
V
REF
0.8V
RUN/SS
V
IN
SLOPE
COMP
Y = “0” ONLY WHEN X IS A CONSTANT “1”
OTHERWISE Y = “1”
FREQ
SHIFT
OSC
2.5µA
g
m
= 0.5m
EA
Ω
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
RUN/
SOFT-START
V
REF
+ 0.12V
SHUTDOWN
V
FB
0.3V
V
IN
V
IN
V
IN
V
CC
ICOMP
I
TH
X
Y
V
IN
0.8V
REFERENCE
UVLO
TRIP = 1.97V
0.8V
V
REF