Datasheet

4
LTC1522
BLOCK DIAGRAM
W
Operation
The LTC1522 uses a switched capacitor charge pump to
boost V
IN
to a regulated 5V ±4% output voltage. Regula-
tion is achieved by sensing the output voltage through an
internal resistor divider and enabling the charge pump
when the output voltage droops below the lower trip point
of COMP1. When the charge pump is enabled, a 2-phase,
nonoverlapping clock controls the charge pump switches.
Clock 1 closes the S1 switches which enables the flying
capacitor to charge up to the V
IN
voltage. Clock 2 closes
the S2 switches that stack C
FLY
in series with V
IN
and
connect the top plate of C
FLY
to the output capacitor at
V
OUT
. This sequence of charging and discharging contin-
ues at a free-running frequency of 700kHz (typ) until the
output has risen to the upper trip point of COMP1 and the
charge pump is disabled. When the charge pump is
disabled, the LTC1522 draws only 4µA (typ) from V
IN
which provides high efficiency at low load conditions.
In shutdown mode, all circuitry is turned off and the part
draws only leakage current from the V
IN
supply. V
OUT
is
also disconnected from V
IN
. The SHDN pin is a CMOS
input with a threshold of approximately V
IN
/2; however,
the SHDN pin can be driven by logic levels that exceed the
V
IN
voltage. The part enters shutdown mode when a logic
high is applied to the SHDN pin. The SHDN pin should not
be floated; it must be driven with a logic high or low.
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1522 will draw
between 100mA and 200mA from V
IN
causing a rise in
the junction temperature. On-chip thermal shutdown
circuitry disables the charge pump once the junction
temperature exceeds ≈160°C, and reenables the charge
pump once the junction temperature falls back to ≈ 145°C.
The LTC1522 will cycle in and out of thermal shutdown
indefinitely without latchup or damage until the V
OUT
short is removed.
Capacitor Selection
For best performance, it is recommended that low ESR
(<0.5Ω) capacitors be used for both C
IN
and C
OUT
to
reduce noise and ripple. The C
IN
and C
OUT
capacitors
should be either ceramic or tantalum and should be 3.3µF
or greater (aluminum capacitors are not recommended
because of their high ESR). If the input source impedance
is very low, C
IN
may not be needed. Increasing the size of
C
OUT
to 10µF or greater will reduce output voltage ripple.
APPLICATIONS INFORMATION
WUU
U
V
REF
CLOCK 1
CLOCK 2
CONTROL
LOGIC
S2A
S2B
S1A
S1B
C
FLY

0.22µF
C
IN

10µF
C
+
C
–
C
OUT

10µF
1µA
V
OUT
SHDN
LTC1522 BD
CHARGE PUMP SHOWN IN DISCHARGE CYCLE
V
IN
CHARGE PUMP
+
+
–
+
COMP1