LTC1522_15 Datasheet, PDF (4/8 Pages) Linear Technology – Micropower, Regulated 5V Charge Pump DC/DC Converter

LTC1522
BLOCK DIAGRAM
V_IN
+
C_IN
10µF
C⁺
C_FLY
0.22µF
C^–
S2A
S1A
S2B
S1B
CHARGE PUMP
CLOCK 1
CLOCK 2
CONTROL
LOGIC
CHARGE PUMP SHOWN IN DISCHARGE CYCLE
COMP1
V_REF
SHDN
+
V_OUT
C_OUT
1µA
10µF
LTC1522 BD
APPLICATIONS INFORMATION
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.
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