AIC1845 Datasheet, PDF (11/13 Pages) List of Unclassifed Manufacturers – Regulated 5V Charge Pump In SOT-23

AIC1845
∆Q = Q_ON - Q_OFF
= C₁ × (V_{C1−ON−AVE} − V_{C1−OFF−AVE} )
= C₁ × (2V_IN - V_OUT - 2R_DS-ON × I_ON-AVE - 2R_DS-ON × I_OFF-AVE - ESR × I_ON−AVE - ESR × I_OFF-AVE )
= C₁ × (2V_IN
− V_OUT
−
2R _{DS −ON}
×
I_OUT
D
−
2R _DS−ON
×
I_OUT
1− D
-
ESR
×
I_OUT
D
- ESR × ^IOUT )
1- D
=
C₁
× [2V_IN
−
V_OUT
− (2R_DS−ON
+ ESR) × I_OUT
×
1]
D(1− D)
………(13)
Thus the output current can be written as
I_OUT = f × ∆Q = f × (Q_ON − Q_OFF )
=
f
× C₁
× [2V_IN
−
V_OUT
-
(2R _DS-ON
+
ESR) × I_OUT
×
1]
D(1- D)
(14)
When the duty cycle is 0.5, the output current can be written as:
I_OUT
=
f × C₁ × [2V_IN
− V_OUT
− (2R_DS−ON
+ ESR) × I_OUT
×
1]
0.5(1− 0.5)
= fC₁ × [2V_IN − V_OUT − (8R_DS−ON + 4ESR) × I_OUT ]
(15)
And equation (15) can be re-written as:
2V_IN
− V_OUT
=
1
fC₁
× I_OUT
+ (8R_DS−ON
+ 4ESR) × I_OUT
(16)
According the equation (16), when the duty cycle
is 0.5, the equivalent circuit of charge pump is
shown in Fig. 22. The term 8R_DS-ON is the total
effect of switching resistance, 1/fC₁ is the effect
of flying capacitor and 4ESR is its equivalent
resistance.
From the equivalent circuit shown in Fig. 22, it is
seen that the terms 1/fC₁, 4ESR and 8R_DS-ON
should be as small as possible to get large output
current. However, for users, since the R_DS-ON is
fixed and manufactured in IC, what we can do is
to lower 1/fC₁ and ESR. However even the effect
of 1/fC₁ and ESR can be kept as small as
possible, the term 8R_DS-ON still dominates the
role that limits the maximum output current.
2V_IN
I_OUT
V_OUT
1/fC1 8RDS-ON 4ESR
C_OUT
LOAD
Fig. 22 The euqivalent circuit of charge pump
Layout Considerations
Due to the switching frequency and high transient
current of AIC1845, careful consideration of PCB
layout is necessary. To achieve the best
performance of AIC1845, minimize the distance
between every two components and also
minimize every connection length with a
maximum trace width. Make sure each device
connects to immediate ground plane. Fig. 23 to
Fig. 25 show the recommended layout.
11