MAX8795A Datasheet, PDF (23/31 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converter with Operational Amplifiers

TFT-LCD DC-DC Converter with
Operational Amplifiers
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:
C_{OUT _ CP}
≥
I_{LOAD _ CP}
2f_OSC V_{RIPPLE _ CP}
where C_{OUT_CP} is the output capacitor of the charge
pump, I_{LOAD_CP} is the load current of the charge
pump, and V_{RIPPLE_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 10kΩ to 30kΩ. Calculate the upper
resistor R4 with the following equation:
⎛
R4 = R5 × _⎝⎜
V_GON
V_FBP
⎞
− 1_⎠⎟
where V_FBP = 1.25V (typ).
Adjust the gate-off linear-regulator REG N output volt-
age by connecting a resistive voltage-divider from
V_GOFF to REF with the center tap connected to FBN
(Figure 1). Select R8 in the 20kΩ to 50kΩ range.
Calculate R7 with the following equation:
R7 = R8 × ^{VFBN − VGOFF}
V_REF − V_FBN
where V_FBN = 250mV, V_REF = 1.25V. Note that REF can
only source up to 50µA; using a resistor less than 20kΩ
for R8 results in higher bias current than REF can supply.
Pass-Transistor Selection
The pass transistor must meet specifications for current
gain (h_FE), input capacitance, collector-emitter saturation
voltage, and power dissipation. The transistor’s current
gain limits the guaranteed maximum output current to:
I_LOAD(MAX)
=
⎛
⎝⎜^IDRV
−
V_BE
R_BE
⎞
⎠⎟
×
h_FE(MIN)
where I_DRV is the minimum guaranteed base-drive cur-
rent, V_BE is the transistor’s base-to-emitter forward volt-
age drop, and R_BE 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 (I_{LOAD(MAX)_LR}) multiplied
by the maximum input-to-output voltage differential:
P = I_{LOAD(MAX)_LR} × (V_{IN(MAX)_LR} − V_{OUT _LR})
where V_{IN(MAX)_LR} is the maximum input voltage of the
linear regulator, and V_OUT__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
_ LR
≅
⎛
⎝⎜
10
V_T
⎞
⎠⎟
×
⎡
⎢1 +
⎣⎢
⎛
⎝⎜
I_BIAS × h_FE
I_{LOAD _ LR}
⎞
⎠⎟
⎤
⎥
⎦⎥
×
V_REF
where V_T is 26mV at room temperature, and I_BIAS is the
current through the base-to-emitter resistor (R_BE). For
the MAX8795A, the bias currents for both the gate-on
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