MAX8795AGCJ/V+T Maxim Integrated Products, MAX8795AGCJ/V+T Datasheet - Page 23

• Manufacturer Part Number: MAX8795AGCJ/V+T
• Description: Display Drivers TFT-LCD DC-DC Conver ter with Operational
• Manufacturer: Maxim Integrated Products

Specifications of MAX8795AGCJ/V+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 C
pump, I
pump, and V
output ripple.
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.
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 10kΩ to 30kΩ. Calculate the upper
resistor R4 with the following equation:
where V
Adjust the gate-off linear-regulator REG N output volt-
age by connecting a resistive voltage-divider from
V
(Figure 1). Select R8 in the 20kΩ to 50kΩ range.
Calculate R7 with the following equation:
where V
only source up to 50μA; using a resistor less than 20kΩ
for R8 results in higher bias current than REF can supply.
The pass transistor must meet specifications for current
gain (h
GOFF
FE
to REF with the center tap connected to FBN
FBN
FBP
OUT_CP
LOAD_CP
), input capacitance, collector-emitter saturation
C
= 1.25V (typ).
= 250mV, V
RIPPLE_CP
OUT CP
R
is the output capacitor of the charge
7
_
R
______________________________________________________________________________________
Linear-Regulator Controllers
is the load current of the charge
=
4
R
=
Charge-Pump Output Capacitor
8
≥
R
Charge-Pump Rectifier Diodes
×
is the peak-to-peak value of the
REF
5
2
V
f
×
OSC
V
FBN
⎛
⎜
⎝
REF
Pass-Transistor Selection
= 1.25V. Note that REF can
V
I
Output-Voltage Selection
V
LOAD CP
GON
FBP
−
V
−
RIPPLE CP
V
V
GOFF
FBN
_
−
1
⎞
⎟
⎠
_
TFT-LCD DC-DC Converter with
voltage, and power dissipation. The transistor’s current
gain limits the guaranteed maximum output current to:
where I
rent, V
age drop, and R
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 (I
by the maximum input-to-output voltage differential:
where V
linear regulator, and V
the linear regulator.
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:
where V
current through the base-to-emitter resistor (R
the MAX8795A, the bias currents for both the gate-on
Operational Amplifiers
P I
BE
=
DRV
A
T
IN(MAX)_LR
LOAD MAX LR
I
V LR
LOAD MAX
is the transistor’s base-to-emitter forward volt-
is 26mV at room temperature, and I
_
is the minimum guaranteed base-drive cur-
(
(
≅
⎛
⎜
⎝
BE
10
V
)_
T
is the maximum input voltage of the
)
⎞
⎟ ×
⎠
=
is the pullup resistor connected
⎛
⎜
⎝
OUT
I
⎡
⎢
⎢
⎣
×
DRV
1
(
+
V
_
IN MAX LR
⎛
⎜
⎝
LR
−
I
(
BIAS
I
LOAD LR
LOAD(MAX)_LR
R
V
Stability Requirements
is the output voltage of
BE
BE
)_
×
⎞
⎟ ×
⎠
_
h
FE
h
−
FE MIN
⎞
⎟
⎠
V
⎤
⎥
⎥
⎦
(
OUT LR
×
V
) multiplied
REF
)
BIAS
_
BE
)
). For
is the
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