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参数资料
| 型号： | MAX8795A+ |
| 厂商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | 1.6 A SWITCHING REGULATOR, 1400 kHz SWITCHING FREQ-MAX, QCC32 |
| 封装： | 5 X 5 MM, 0.80 MM HEIGHT, LEAD FREE, MO-220WHHD-2, TQFN-32 |
| 文件页数： | 10/24页 |
| 文件大小： | 387K |
| 代理商： | MAX8795A+ |
MAX8795A
TFT-LCD DC-DC Converter with
Operational Amplifiers
1
18
8
_
__
_
Charge Pumps
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirement. Figures 8 and 9 show the positive and
negative charge-pump output voltages for a given
VMAIN for one-, two-, and three-stage charge pumps.
The number of positive charge-pump stages is given by:
where nPOS is the number of positive charge-pump
stages, VGON is the gate-on linear-regulator REG P out-
put, VMAIN is the main step-up regulator output, VD is
the forward-voltage drop of the charge-pump diode,
and VDROPOUT is the dropout margin for the linear reg-
ulator. Use VDROPOUT = 0.3V.
The number of negative charge-pump stages is given by:
where nNEG is the number of negative charge-pump
stages, VGOFF is the gate-off linear-regulator REG N
output, VMAIN is the main step-up regulator output, VD
is the forward-voltage drop of the charge-pump diode,
and VDROPOUT is the dropout margin for the linear reg-
ulator. Use VDROPOUT = 0.3V.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to VMAIN and the first stage of the
negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for bet-
ter efficiency. This can be done by connecting the first
stage to VIN or another available supply. If the first
charge-pump stage is powered from VIN, the above
equations become:
Flying Capacitors
Increasing the flying-capacitor (CX) value lowers the
effective source impedance and increases the output-
current capability. Increasing the capacitance indefinitely
has a negligible effect on output-current capability
because the internal switch resistance and the diode
impedance place a lower limit on the source imped-
ance. A 0.1F ceramic capacitor works well in most
low-current applications. The flying capacitor’s voltage
rating must exceed the following:
Vn V
CX
MAIN
>×
n
VV
V
VV
n
VV
V
VV
POS
GON
DROPOUT
IN
MAIN
D
NEG
GOFF
DROPOUT
IN
MAIN
D
=
++
×
= +
+
×
2
n
VV
NEG
GOFF
DROPOUT
MAIN
D
= +
×
2
n
VV
V
VV
POS
GON
DROPOUT
MAIN
D
=
+
×
2
POSITIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. VMAIN
VMAIN (V)
G_ON
(V)
12
10
8
6
4
10
20
30
40
50
60
0
214
2-STAGE CHARGE PUMP
3-STAGE CHARGE PUMP
VD = 0.3V TO 1V
1-STAGE CHARGE PUMP
Figure 8. Positive Charge-Pump Output Voltage vs. VMAIN
NEGATIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. VMAIN
VMAIN (V)
G_OFF
(V)
12
10
8
6
4
-40
-35
-30
-25
-20
-15
-10
-5
-0
-45
214
1-STAGE
CHARGE PUMP
2-STAGE
CHARGE PUMP
3-STAGE
CHARGE PUMP
VD = 0.3V TO 1V
Figure 9. Negative Charge-Pump Output Voltage vs. VMAIN
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