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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： | 11/24页 |
| 文件大小： | 387K |
| 代理商： | MAX8795A+ |
MAX8795A
TFT-LCD DC-DC Converter with
Operational Amplifiers
_
__
_
1
19
9
where n is the stage number in which the flying capaci-
tor appears, and VMAIN is the output voltage of the
main step-up regulator.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak transient voltage. With ceramic capacitors, the
output voltage ripple is dominated by the capacitance
value. Use the following equation to approximate the
required capacitor value:
where COUT_CP is the output capacitor of the charge
pump, ILOAD_CP is the load current of the charge
pump, and VRIPPLE_CP is the peak-to-peak value of the
output ripple.
Charge-Pump Rectifier Diodes
Use low-cost silicon switching diodes with a current rat-
ing equal to or greater than two times the average
charge-pump input current. If it helps avoid an extra
stage, some or all of the diodes can be replaced with
Schottky diodes with an equivalent current rating.
Linear-Regulator Controllers
Output-Voltage Selection
Adjust the gate-on linear-regulator (REG P) output volt-
age by connecting a resistive voltage-divider from the
REG P output to AGND with the center tap connected
to FBP (Figure 1). Select the lower resistor of the divider
R5 in the range of 10k
to 30k. Calculate the upper
resistor R4 with the following equation:
where VFBP = 1.25V (typ).
Adjust the gate-off linear-regulator REG N output volt-
age by connecting a resistive voltage-divider from
VGOFF to REF with the center tap connected to FBN
(Figure 1). Select R8 in the 20k
to 50k range.
Calculate R7 with the following equation:
where VFBN = 250mV, VREF = 1.25V. Note that REF can
only source up to 50A; using a resistor less than 20k
for R8 results in higher bias current than REF can supply.
Pass-Transistor Selection
The pass transistor must meet specifications for current
gain (hFE), input capacitance, collector-emitter saturation
voltage, and power dissipation. The transistor’s current
gain limits the guaranteed maximum output current to:
where IDRV is the minimum guaranteed base-drive cur-
rent, VBE is the transistor’s base-to-emitter forward volt-
age drop, and RBE is the pullup resistor connected
between the transistor’s base and emitter. Furthermore,
the transistor’s current gain increases the linear regula-
tor’s DC loop gain (see the
Stability Requirements sec-
tion), so excessive gain destabilizes the output.
Therefore, transistors with current gain over 100 at the
maximum output current can be difficult to stabilize and
are not recommended unless the high gain is needed to
meet the load-current requirements.
The transistor’s saturation voltage at the maximum out-
put current determines the minimum input-to-output
voltage differential that the linear regulator can support.
Also, the package’s power dissipation limits the usable
maximum input-to-output voltage differential. The maxi-
mum power-dissipation capability of the transistor’s
package and mounting must exceed the actual power
dissipated in the device. The power dissipated equals
the maximum load current (ILOAD(MAX)_LR) multiplied
by the maximum input-to-output voltage differential:
where VIN(MAX)_LR is the maximum input voltage of the
linear regulator, and VOUT_LR is the output voltage of
the linear regulator.
Stability Requirements
The MAX8795A linear-regulator controllers use an inter-
nal transconductance amplifier to drive an external
pass transistor. The transconductance amplifier, the
pass transistor, the base-emitter resistor, and the out-
put capacitor determine the loop stability. The following
applies to both linear-regulator controllers in the
MAX8795A.
The transconductance amplifier regulates the output
voltage by controlling the pass transistor’s base cur-
rent. The total DC loop gain is approximately:
A
V
Ih
I
V
VLR
T
BIAS
FE
LOAD LR
REF
_
×+
×
×
10
1
PI
V
LOAD MAX
LR
IN MAX
LR
OUT LR
=×
() _
(
) _
_
()
II
V
R
h
LOAD MAX
DRV
BE
FE MIN
()
(
)
=
×
RR
VV
FBN
GOFF
REF
FBN
78
=×
RR
V
GON
FBP
45
1
=×
C
I
fV
OUT CP
LOAD CP
OSC
RIPPLE CP
_
≥
2
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