AN023 Datasheet, PDF (1/6 Pages) Analog Intergrations Corporation – An Useful Model for Charge Pump Converter

AN023
An Useful Model for Charge Pump Converter
Introduction
Analysis
This application note focuses on the analysis of
the charge pump and its equivalent circuit. The
AIC1845 is a micro-power charge pump DC/DC
converter that produces a regulated 5V output. This
kind of converter uses capacitors to store and transfer
energy. Since the capacitors can not 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.1, 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, 2 V_IN.
V_IN
C_IN
I_{ON Q1}
Q2
R_{DS-ON ESR}
V_{OU T}
C_OUT
Q3
C1 _Q4
R_DS-ON
Fig.2 The on State of Charge Pump Circuit
V_IN
C_IN
R_DS-ON
Q1
Q2
V_{OU T}
ESR ^COUT
Q3
R_DS-ON
I_OFF
C1 _Q4
Fig.3 The OFF State of Charge Pump Circuit
V_IN
Q1
C_IN
Q2
V_OUT
C1
C_OUT
Q3
Q4
Fig.1 The Circuit of Charge Pump
Referring to Fig.2 and Fig.3, 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.
I_ON−AVE × DT = I_OFF−AVE × (1− D)T
(1)
I_ON-AVE × D = I_OFF-AVE × (1− D)
(2)
February, 2002
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