AIC1845 Datasheet, PDF (9/13 Pages) List of Unclassifed Manufacturers – Regulated 5V Charge Pump In SOT-23

AIC1845
I_ON−AVE × DT = I_OFF−AVE × (1 − D)T
(1)
I_ON-AVE × D = I_OFF-AVE × (1− D)
(2)
I_IN = I_ON-AVE × D + I_OFF-AVE × (1− D)
= 2 × I_ON-AVE × D
(3)
= 2 × I_OFF-AVE × (1- D)
I_OUT = I_OFF-AVE × (1− D)
I_IN = 2I_OUT
For AIC1845, the controller takes the PSM (Pulse
Skipping Modulation) control strategy. When the
duty cycle is limited to 0.5, there will be:
I_ON-AVE × 0.5 × T = I_OFF-AVE × (1− 0.5) × T
I_ON-AVE = I_OFF-AVE
According to the equation (4), we know that as
long as the flying capacitor C1 is at steady state,
the input current is twice the output current. The
efficiency of charge pump is given below:
η = ^{VOUT × IOUT} = ^{VOUT × IOUT} = ^VOUT
V_IN × I_IN
V_IN × 2I_OUT 2V_IN
V_IN
ION
Q1
Q2 _VOUT
R_DS-ON
ESR
C_IN
C_OUT
Q3
C1 _Q4
R_DS-ON
Fig. 20 The on state of charge pump circuit
V_IN
C_IN
R_DS-ON
Q1
Q3 ESR
Q2 _VOUT
C_OUT
R_DS-ON
Q4
C1
I_OFF
Fig. 21 The off state of charge pump circuit
External Capacitor Selection
Three external capacitors, C_IN, C_OUT and C_FLY,
determine AIC1845 performances, in the aspects
of output ripple voltage, charge pump strength
and transient. Optimum performance can be
obtained by the use of ceramic capacitors with
low ESR. Due to high ESR, capacitors of tantalum
and aluminum are not recommended for charge
pump application.
To reduce noise and ripple, a low ESR ceramic
capacitor, ranging from 2.2µF to 10µF, is
recommended for C_IN and C_OUT. The value of
C_OUT determines the amount of output ripple
voltage. An output capacitor with larger value
results in smaller ripple.
.......... (5)
C_FLY is critical for the strength of charge pump.
The larger C_FLY is, the larger output current and
smaller ripple voltage obtain. However, large C_IN
and C_OUT are expected when a large.C..._F^._L^.._Y^{..a.(p6p)lies.}
The ratio of C_IN (as well as C_OUT) to C_FLY should
be approximately 10:1.
The value of capacitors, which is used under
operation conditioin, determines the performance
of a charge pump converter. And two factors, as
follows, affect the capacitance of capacitor.
1. Material: Ceramic capacitors of different
materials, such as X7R, X5R, Z5U and Y5V,
have different tolerance in temperature and
differnet cpacitance loss. For example, a X7R
or X5R type of capacitor can retain most of
the capacitance at temperature from -40°C to
85°C, but a Z5U or Y5V type will lose most of
the capacitance at that temperature range.
9