LTC3240-3.3_15 Datasheet, PDF (7/12 Pages) Linear Technology – 3.3V/2.5V Step-Up/ Step-Down Charge Pump DC/DC Converter

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OPERATIO (Refer to the Block Diagram)
The LTC3240 is a step-up/step-down charge pump DC/DC
converter. For V_IN greater than V_OUT by about 100mV it
operates as a low dropout regulator. Once V_IN drops to
within 100mV of V_OUT, the part automatically switches
into charge pump mode to boost V_IN to the regulated
output voltage. Regulation is achieved by sensing the
output voltage through an internal resistor divider and
modulating the charge pump output current based on
the error signal.
In the charge pump mode a 2-phase nonoverlapping clock
activates the charge pump switches. The flying capacitor
is charged from V_IN on the first phase of the clock. On
the second phase of the clock it is stacked in series with
V_IN and connected to V_OUT. This sequence of charging
and discharging the flying capacitor continues at a free
running frequency of 1.2MHz (typ).
Shutdown Mode
In shutdown mode, all circuitry is turned off and the
LTC3240 draws only leakage current from the V_IN supply.
Furthermore, V_OUT is disconnected from V_IN. The ^⎯S^⎯H^⎯D^⎯N
pin is a CMOS input with a threshold voltage of approxi-
mately 0.8V. The LTC3240 is in shutdown when a logic
low is applied to the ^⎯S^⎯H^⎯D^⎯N pin. Since the ^⎯S^⎯H^⎯D^⎯N pin is a
high impedance CMOS input, it should never be allowed
to float. To ensure that its state is defined, it must always
be driven with a valid logic level.
Since the output voltage of this device can go above the
input voltage, circuitry is required to control the state of
the converter even in shutdown. This circuitry will draw
an input current of 5μA in shutdown. However, this cur-
rent is eliminated when the output voltage (V_OUT) drops
to less than approximately 0.8V.
Burst Mode Operation
The LTC3240 provides automatic Burst Mode operation
while operating as a charge pump, to increase efficiency of
the power converter at light loads. Burst Mode operation
LTC3240-3.3/LTC3240-2.5
is initiated if the output load current falls below an inter-
nally programmed threshold. Once Burst Mode operation
is initiated, the part shuts down the internal oscillator to
reduce the switching losses, and goes into a low current
state. This state is referred to as the sleep state in which
the IC consumes only about 65μA from the input. When
the output voltage droops enough to overcome the burst
comparator hysteresis, the part wakes up and commences
normal fixed frequency operation recharging the output
capacitor. If the output load is still less than the Burst Mode
threshold the part will re-enter sleep state. This Burst Mode
threshold varies with V_IN, V_OUT and the choice of output
storage capacitor.
Soft-Start
The LTC3240 has built-in soft-start circuitry to prevent
excessive current flow during start-up. The soft-start is
achieved by internal circuitry that slowly ramps the amount
of current available to the output storage capacitor from
zero to a value of 300mA over a period of approximately
2ms. The soft-start circuitry is reset in the event of a com-
manded shutdown or thermal shutdown.
Short-Circuit/Thermal Protection
The LTC3240 has built-in short-circuit current limit as
well as overtemperature protection. During a short-circuit
condition, the part automatically limits its output current to
approximately 300mA. If the junction temperature exceeds
approximately 160°C the thermal shutdown circuitry shuts
down current delivery to the output. Once the junction
temperature drops back to approximately 150°C cur-
rent delivery to the output is resumed. The LTC3240 will
cycle in and out of thermal shutdown indefinitely without
latch-up or damage until the short-circuit condition on
V_OUT is removed. Long term overstress (i.e. operation at
junction temperatures above 125°C) should be avoided
as it reduces the lifetime of the part and can result in
degraded performance.
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