Datasheet

Flying Capacitors
Increasing the flying capacitor value lowers the effec-
tive 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 impedance. A 0.1μF ceramic
capacitor works well in most low-current applications.
The voltage rating for a given flying capacitor (CX) must
exceed the following:
V
CX
> N x V
IN
where N is the stage number in which the flying capaci-
tor appears, and V
IN
is the input voltage of the stepdown
regulator.
Charge-Pump Output Capacitors
Increasing the output capacitance or decreasing the ESR
reduces the charge pump 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:
LOAD
OUT
OSC RIPPLE
2f V
≥
where V
RIPPLE
is the peak-to-peak value of the output
ripple.
Charge-Pump Rectier Diodes
Use low-cost silicon switching diodes with a current rating
equal to or greater than 2 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 positive linear regulator (LR1 to LR4) output
voltages by connecting a resistive voltage-divider from the
output to AGND with the center tap connected to FBL_
(Figure 1). Select the lower resistor of the divider in the
10kΩ to 30kΩ range. Calculate the upper resistor with the
following equation:
( )
UPPER LOWER OUT_ FBL_
R R V /V 1

=×−

where V
FBL_
is 1.238V (typ).
Adjust the negative linear regulator (LR5) output voltage
by connecting a resistive voltage-divider from V
GOFF
to VL with the center tap connected to FBL5 (Figure 1).
Select R29 in the 10kΩ to 30kΩ range.Calculate R28 with
the following equation:
( )
FBL5 GOFF L FBL5
R28 R29 V V ) / (V V

=×− −

where V
FBL5
= 125mV and V
L
= 5.0V.
Pass Transistor Selection
The pass transistor must meet specifications for DC cur-
rent gain (hFE), collector-emitter saturation voltage, and
power dissipation. The transistor’s current gain limits the
guaranteed maximum output current to:
BE
LOAD(MAX) DRV FE
BE
V
IIh
R

= −×


where I
DRV
is the minimum guaranteed base drive cur-
rent, V
BE
is the base-emitter voltage of the pass transis-
tor, and R
BE
is the pullup resistor connected between the
transistor’s base and emitter. Furthermore, the transis-
tor’s current gain increases the linear regulator’s DC loop
gain (see the Stability Requirements section), which may
destabilize the output. Therefore, transistors with current
gain over 300 at the maximum output current can be diffi-
cult 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 output
current determines the minimum input-to-output voltage
differential that the linear regulator supports. Also, the
package’s power dissipation limits the usable maximum
input-to-output voltage differential. The maximum power
dissipation capability of the transistor’s package and
mounting must exceed the actual power dissipation in the
device. The power dissipation equals the maximum load
current (I
LOAD(MAX)
) times the maximum input-to-output
voltage differential:
LOAD(MAX) LRIN(MAX) LROUT
P I (V V )=×−
where V
LRIN(MAX)
is the maximum input voltage of the
linear regulator, and V
LROUT
is the output voltage of the
linear regulator.
Output Voltage Ripple
Ideally, the output voltage of a linear regulator should
not contain any ripple. In the MAX1530/MAX1531, the
step-down regulator’s switching noise can couple to the
linear regulators, creating output voltage ripple. Following
the PC board layout guidelines in the PC Board Layout
and Grounding section can significantly reduce noise
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
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