AIC1845 Datasheet, PDF (8/13 Pages) List of Unclassifed Manufacturers – Regulated 5V Charge Pump In SOT-23

AIC1845
APPLICATION INFORMATION
Introduction
AIC1845 is a micropower charge pump DC/DC
converter that produces a regulated 5V output
with an input voltage range from 2.7V to 5.0V. It
utilizes the charge pump topology to boost V_IN to
a regulated output voltage. Regulation is obtained
by sensing the output voltage through an internal
resistor divider. A switched doubling circuit
enables the charge pump when the feedback
voltage is lower than the trip point of the internal
comparator, and vice versa. When the charge
pump is enabled, a two-phase non-overlapping
clock activates the charge pump switches. To
maximize battery life for a battery-used application,
quiescent current is limited up to 13µA.
Operation
This kind of converter uses capacitors to store
and transfer energy. Since the capacitors can’t
change their voltage level abruptly, the voltage
ratio of V_OUT over V_IN is limited to some range.
Capacitive voltage conversion is obtained by
switching a capacitor periodically. It first charges
the capacitor by connecting it across a voltage
source and then connects it to the output.
Referring to Fig. 19, during the on state of internal
clock, Q₁ and Q₄ are closed, which charges C₁ to
V_IN level. During the off state, Q₃ and Q₂ are
closed. The output voltage is V_IN plus V_C1, that is,
2V_IN.
V_IN
Q1
Q2 VOUT
C_IN
C1
C_OUT
Q3
Q4
Fig. 19 The circuit of charge pump
Short Circuit/Thermal Protection
AIC1845 owns a built-in short circuit current
limiting as well as an over temperature protection.
During the short circuit condition, the output
current is automatically constrained at
approximately 170mA. This short circuit current
will cause a rise in the internal IC junction
temperature. When the die temperature exceeds
150°C, the thermal protection will shut the charge
pump switching operation down and the die
temperature will reduce afterwards. Once the die
temperature drops below 135°C, the charge pump
switching circuit will re-start. If the fault doesn’t
eliminate, the above protecting operation will
repeat again and again. It allows AIC1845 to
continuously work at short circuit condition without
damaging the device.
Shutdown
In shutdown mode, the output is disconnected
from input. The input current gets extremely low
since most of the circuitry is turned off. Due to
high impedance, shutdown pin can’t be floated.
Efficiency
Referring to Fig. 20 and Fig. 21 here shows the
circuit of charge pump at different states of
operation. R_DS-ON is the resistance of the switching
element at conduction. ESR is the equivalent
series resistance of the flying capacitor C₁. I_ON-
AVE ^{and I}OFF-AVE ^{are the average current during on}
state and off state, respectively. D is the duty
cycle, which means the proportion the on state
takes. Let’s take advantage of conversation of
charge for capacitor C₁. Assume that the
capacitor C₁ has reached its steady state. The
amount of charge flowing into C₁ during on state
is equal to that flowing out of C₁ at off state.
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