LTC3204-5_15 Datasheet, PDF (7/12 Pages) Linear Technology – Low Noise Regulated Charge Pump in 2 2 DFN

U
OPERATIO (Refer to the Block Diagram)
The LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
use a switched capacitor charge pump to boost V_IN to a
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. 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
LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
draws only leakage current from the V_IN supply. Further-
more, V_OUT is disconnected from V_IN. The SHDN pin is a
CMOS input with a threshold voltage of approximately 0.7V.
The LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
are in shutdown when a logic low is applied to the SHDN
pin. Since the SHDN pin is a very 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 voltages of these devices can go above
the input voltage, special circuitry is required to control
the internal logic. Detection logic will draw an input current
of 5µA when the devices are in shutdown. However, this
current will be eliminated if the output voltage (V_OUT) is
less than approximately 0.8V.
Burst Mode Operation
The LTC3204-3.3/LTC3204-5 provide automatic Burst
Mode operation to reduce supply current at light loads.
Burst Mode operation is initiated if the output load current
falls below an internally programmed threshold. Once
LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5
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 40µA
from the input. When the output voltage droops enough
to overcome the burst comparator hysteresis, the part
wakes up and commences normal fixed frequency opera-
tion. The output capacitor recharges and causes the part
to reenter the sleep state if the output load still remains
less than the Burst Mode threshold. This Burst Mode
threshold varies with V_IN, V_OUT and the choice of output
storage capacitor.
Soft-Start
The LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
have built-in soft-start circuitry to prevent excessive current
flow during start-up. The soft-start is achieved by charging
an internal capacitor with a very weak current source. The
voltage on this capacitor, in turn, slowly ramps the amount
of current available to the output storage capacitor from
zero to a value of 300mA over a period of approximately
0.75ms. The soft-start circuit is reset in the event of a
commanded shutdown or thermal shutdown.
Short-Circuit/Thermal Protection
The LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5
have built-in short-circuit current limit as well as over-tem-
perature protection. During a short-circuit condition, they
will automatically limit their output current to approximately
300mA. At higher temperatures, or if the input voltage is
high enough to cause excessive self-heating of the part,
the thermal shutdown circuitry will shutdown the charge
pump once the junction temperature exceeds approximately
160°C. It will enable the charge pump once the junction
temperature drops back to approximately 150°C. The
LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 will
cycle in and out of thermal shutdown indefinitely without
latchup or damage until the short-circuit condition on
V_OUT is removed.
3204fa
7