LTC3221_15 Datasheet, PDF (7/12 Pages) Linear Technology – Micropower, Regulated Charge Pump

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OPERATIO (Refer to Block Diagrams)
The LTC3221 has a FB pin in place of the ⎯S⎯H⎯D⎯N pin. This
allows the output voltage to be programmed using an
external resistive divider.
Burst Mode Operation
The LTC3221 family regulates the output voltage throughout
the full 60mA load range using Burst Mode control. This
keeps the quiescent current low at light load and improves
the efficiency at full load by reducing the switching losses.
All the internal circuitry except the comparator is kept off
if the output voltage is high and the flying capacitor has
been fully charged. These circuits are turned on only if V_OUT
drops below the comparator lower threshold. At light load,
LTC3221/
LTC3221-3.3/LTC3221-5
V_OUT stays above this lower threshold for a long period of
time, this result in a very low average input current.
Soft-Start and Short-Circuit Protection
The LTC3221 family uses a controlled current, I_SW to
deliver current to the output. This helps to limit the input
and output current during start-up and output short-circuit
condition. During start up I_SW is used to charge up the flying
capacitor and output capacitor, this limits the input current
to approximately 240mA. During short-circuit condition,
the output current is delivered through I_SW and this limits
the output current to approximately 120mA. This prevents
excessive self-heating that causes damage to the part.
APPLICATIO S I FOR ATIO
Power Efficiency
The input current of a doubling charge pump like the LTC3221
family is always twice that of the output current. This is
true regardless of whether the output voltage is unregulated
or regulated or of the regulation method used. In an ideal
unregulated doubling charge pump, conservation of energy
implies that the input current has to be twice that of the
output current in order to obtain an output voltage twice
that of the input voltage. In a regulated charge pump like
the LTC3221, the regulation of V_OUT is similar to that of a
linear regulator, with the voltage difference between 2 • V_IN
(Input voltage plus the voltage across a fully charged flying
capacitor) and V_OUT being absorbed in an internal pass
transistor. In the LTC3221, the controlled current I_SW acts as
a pass transistor. So the input current of an ideal regulated
doubling charge pump is the same as an unregulated one,
which is equal to twice the output current. The efficiency
(n) of an ideal regulated doubler is therefore given by:
η = ^POUT = ^{VOUT •IOUT} = ^VOUT
P_IN V_IN • 2I_OUT 2V_IN
At moderate to high output power, the switching losses and
quiescent current of the LTC3221 family are negligible and
the expression is valid. For example, an LTC3221-5 with V_IN
= 3V, I_OUT = 60mA and V_OUT regulating to 5V, has a mea-
sured efficiency of 82% which is in close agreement with
the theoretical 83.3% calculation. The LTC3221 product
family continues to maintain good efficiency even at fairly
light loads because of its inherently low power design.
Maximum Available Output Current
For the adjustable LTC3221, the maximum available output
current and voltage can be calculated from the effective
open-loop output resistance, R_OL, and effective output
voltage, 2V_IN(MIN).
From Figure 1 the available current is given by:
I_OUT
=
2V_IN – V_OUT
R_OL
Effective Open-Loop Output Resistance (R_OL)
The effective open-loop output resistance(R_OL) of a charge
pump is a very important parameter which determines the
strength of the charge pump. The value of this parameter
R_OL
–+ ^2VIN
3221 F01
+
I_OUT V_OUT
–
Figure 1. Equivalent Open-Loop Circuit
3221f
7