AIC1533
For negative charge pump, the number of negative
charge pump can be determined by
NNEG
≥
RTH × Iout − VNEG
Vcc − 2VD
(6)
where VNEG = output of negative charge pump
Example
To obtain VPOS=28V/30mA, VNEG=-9V/30mA from
VCC=12V, CFLY=0.1µF, the number of positive and
negative charge pumps can be calculated as
follows
RTH
=
2(R DS,ON(P )
+ RDS,ON(N) )
+
(
1
CFLY × FSW
)+(
CO
1
× FSW
)
= 2(10 + 10) + (
1
)+(
1
)
0.1× 10 −6 × 1× 106 1× 10 −6 × 1× 106
=51
(7)
NPOS
≥ VPOS − Vcc + R TH × Iout = 28 − 12 + (51× 30 × 10 −3 )
Vcc − 2VD
12 − 2 × 0.5
= 1.59
(8)
NNEG
≥ R TH × Iout − VNEG = (51× 30 × 10 −3 ) − (−9) (9)
Vcc − 2VD
12 − 2 × 0.5
= 0.96
Thus, the number of positive charge pump is two,
and the negative one requires one charge pump.
2. Output Voltage Selection
2.1 Output voltage of LDO
A resistor divider, determining output voltage of
LDO, connects to OUT, GND, and ADJ (referred to
TYPICAL APPLICATION CIRCUIT). Values of
resistors can be calculated by equation (10).
VLDO
=
VADJ
× (1+
R3 )
R4
(10)
where VLDO = output voltage of LDO
VADJ = adjustable voltage of LDO, the
recommended R4 is 130kΩ.
2.2. Output Voltage Of Positive Charge
Pump
Output voltage of positive charge pump is
determined by connecting a resistor divider to
output terminal of positive charge pump, GND, and
FBP (referred to TYPICAL APPLICATION
CIRCUIT). The values of resistor divider can be
calculated as equation (11).
VPOS
=
VFBP
× (1+
R1 )
R2
(11)
where VPOS = output voltage of positive charge
pump
VFBP = feedback voltage of positive charge pump,
1.25V. The recommended R2 is 22kΩ
2.3 Output voltage of negative charge
pump
Since comparator input of negative charge pump,
FBN, is referenced to 0V, a positive reference
voltage, which can be obtained by adding a 0.22µF
bypass capacitor between VREF and GND, is
needed (referred to TYPICAL APPLICATION
CIRCUIT). The value of resistor dividers can be
calculated as equation (12).
VNEG
=
−VREF
×
R5
R6
(12)
where VNEG represents output voltage of negative
charge pump, and VREF means reference voltage,
1.25V. The recommended R6 is 33kΩ.
13