MAX1513 Datasheet, PDF (25/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers

TFT-LCD Power-Supply Controllers
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_OSCV_{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.
The charge-pump output capacitor is typically also the
input capacitor for a linear regulator. Often, its value must
be increased to maintain the linear regulator’s stability.
Charge-Pump Rectifier Diodes
Use low-cost silicon switching diodes with a current rating
equal to or greater than twice the average charge-pump
input current. If their low forward voltage helps to avoid an
extra stage, some or all of the diodes can be replaced
with Schottky diodes with equivalent current ratings.
Linear-Regulator Controllers
Output-Voltage Selection
Adjust the positive linear-regulator (REG P, REG L, and
REG G) output voltages by connecting a resistive volt-
age-divider from their respective outputs to the analog
ground plane (which connects to GND) with the center
tap connected to FB_ (Figure 1). Select the lower resis-
tor of the divider in the range of 10kΩ to 30kΩ. Calculate
the upper resistor with the following equation:
R_UPPER
=
R_LOWER
×
⎛ _{VOUT _}
⎝⎜ ^VFB_
⎞
- 1_⎠⎟
where V_OUT_ is the output voltage of the respective lin-
ear regulator, and V_FB_ = 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 R4 between 20kΩ and 50kΩ.
Calculate R3 with the following equation:
R3
=
R4
×
⎛
⎝⎜
V_FBN
V_REF
-
-
V_GOFF
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 R4 results in higher bias current than REF can sup-
ply without degrading REF accuracy.
Pass-Transistor Selection
The pass transistor must meet specifications for current
gain (h_FE), input capacitance, collector-emitter satura-
tion voltage, and power dissipation. The transistor’s
current gain limits the guaranteed maximum output cur-
rent to:
I_LOAD(MAX) =
⎛
⎝⎜^{IDRV -}
V_BE
R_BE
⎞
⎠⎟
×
h_FE(MIN)
where I_DRV is the minimum guaranteed base-drive cur-
rent and R_BE is the pullup resistor connected between
the transistor’s base and emitter. Furthermore, the tran-
sistor’s current gain increases the linear regulator’s DC
loop gain (see the Stability Requirements section), so
excessive gain destabilizes the output. Therefore, tran-
sistors with current gain over 100 at the maximum out-
put current can be difficult to stabilize and are not
recommended unless needed to meet output-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 supports.
Also, the package’s power dissipation limits the use-
able maximum power-dissipation capability of the tran-
sistor’s package, and mounting must exceed the actual
power dissipation in the device. The power dissipation
equals the maximum load current (I_{LOAD(MAX)_LR})
times 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 MAX1513/MAX1514 linear-regulator controllers use
an internal transconductance amplifier to drive an
external pass transistor. The transconductance amplifi-
er, the pass transistor, the base-emitter resistor, and
the output capacitor determine the loop stability. The
following applies equally to all linear regulators in the
MAX1513 and MAX1514.
The transconductance amplifier regulates the output
voltage by controlling the pass transistor’s base cur-
rent. The total DC loop gain is approximately:
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