LTC3203_15 Datasheet, PDF (8/16 Pages) Linear Technology – 500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps

LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
OPERATIO
The LTC3203/LTC3203-1/LTC3203B/LTC3203B-1 use
a switched capacitor charge pump to boost V_IN to a
regulated output voltage. Regulation is achieved by sens-
ing the output voltage through a resistor divider and
modulating the charge pump output current based on the
error signal. A two-phase non-overlapping clock activates
the charge pump switches. The typical frequency of charg-
ing and discharging the flying capacitors is 1MHz
(2x mode) or 0.9MHz (1.5x mode). A unique architecture
maintains relatively constant input current for the lowest
possible input noise.
Mode of Operation
The LTC3203/LTC3203-1/LTC3203B/LTC3203B-1 charge
pump can operate in two modes of voltage conversion:
1.5x or 2x.
In the 1.5x mode the flying capacitors are charged in series
during the first clock phase, and stacked in parallel on top
of V_IN on the second clock phase. Alternatively, in the 2x
mode the flying capacitors are charged on alternate clock
phases from V_IN. While one capacitor is being charged
from V_IN, the other is stacked on top of V_IN and connected
to the output. The two flying capacitors operate out of
phase to minimize both input and output ripple. At light
load the LTC3203/LTC3203-1 go into Burst Mode opera-
tion to reduce quiescent current.
The conversion mode should be chosen based on consid-
erations of efficiency, available output current and V_OUT
ripple. With a given V_IN, the 1.5x mode gives a higher
efficiency but lower available output current. The 2x mode
gives a higher available output current but lower effi-
ciency. Moreover, the output voltage ripple in the 2x mode
is lower due to the out-of-phase operation of the two
flying capacitors.
Generally, at low V_IN, the 2x mode should be selected, and
at higher V_IN, the 1.5x mode should be selected. By
connecting a resistive divider from V_IN to the MODE input
pin the MODE input allows the user to accurately program
the V_IN threshold at which the charge pump will switch
from 1.5x mode to 2x mode when V_IN starts to fall and
vice versa. Hysteresis on the MODE pin prevents the
LTC3203/LTC3203-1/LTC3203B/LTC3203B-1 from
switching continuously between the two modes.
Output Voltage Programming
The LTC3203-1/LTC3203B-1 has a V_SEL input pin that
allows the user to program the regulated output voltage to
either 4.5V or 5V. 4.5V V_OUT is useful for driving white
LEDs while a regulated V_OUT of 5V is useful for powering
logic circuits.
The LTC3203/LTC3203B has a FB pin in place of the V_SEL
pin that allows the output voltage to be programmed using
an external resistive divider.
Shutdown Mode
When SHDN is asserted low, the LTC3203/LTC3203-1/
LTC3203B/LTC3203B-1 enter shutdown mode. The charge
pump is first disabled, but the LTC3203/LTC3203-1/
LTC3203B/LTC3203B-1 continue to draw 5µA of supply
current. This current will drop to less than 1µA when V_OUT
is fully discharged to 0V. Furthermore, V_OUT is discon-
nected from V_IN. Since the SHDN pin is a high impedance
CMOS input, it should never be allowed to float.
Burst Mode Operation
The LTC3203/LTC3203-1 provide automatic Burst Mode
operation to reduce quiescent current of the power con-
verter at light loads. Burst Mode operation is initiated if the
output load current falls below an internally 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 chip consumes
only about 120µA from the input.
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