ltc3240-3.3 Linear Technology Corporation, ltc3240-3.3 Datasheet - Page 7

• Manufacturer Part Number: ltc3240-3.3
• Description: 3.3v/2.5v Step-up/ Step-down Charge Pump Dc/dc Converter
• Manufacturer: Linear Technology Corporation

OPERATIO
The LTC3240 is a step-up/step-down charge pump DC/DC
converter. For V
operates as a low dropout regulator. Once V
within 100mV of V
into charge pump mode to boost V
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 fl ying capacitor
is charged from V
the second phase of the clock it is stacked in series with
V
and discharging the fl ying 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
Furthermore, V
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 fl oat. To ensure that its state is defi ned, 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
to less than approximately 0.8V.
Burst Mode Operation
The LTC3240 provides automatic Burst Mode operation
while operating as a charge pump, to increase effi ciency of
the power converter at light loads. Burst Mode operation
IN
and connected to V
OUT
IN
U
IN
greater than V
OUT
is disconnected from V
on the fi rst phase of the clock. On
(Refer to the Block Diagram)
, the part automatically switches
OUT
. This sequence of charging
OUT
IN
by about 100mV it
to the regulated
IN
. The ⎯ S ⎯ H ⎯ D ⎯ N
IN
OUT
IN
drops to
) drops
supply.
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 fi xed 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
storage capacitor.
Soft-Start
The LTC3240 has built-in soft-start circuitry to prevent
excessive current fl ow 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 indefi nitely without
latch-up or damage until the short-circuit condition on
V
junction temperatures above 125°C) should be avoided
as it reduces the lifetime of the part and can result in
degraded performance.
OUT
LTC3240-3.3/LTC3240-2.5
is removed. Long term overstress (i.e. operation at
IN
, V
OUT
and the choice of output
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