Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
30 _____________________________________________________________________________________
Output Capacitor Selection
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s ESR:
GOFF2_RIPPLE GOFF2_RIPPLE(C)
GOFF2_RIPPLE(ESR)
V V
V
=
+
( )
GOFF2
GOFF2
GOFF2_RIPPLE(C)
GOFF2 SW IN3 GOFF2
-V
I
V
C f (V - V )
≈ ×
and:
GOFF2_RIPPLE(ESR) GOFF2_PEAK ESR_AVDD
V I R≈
where I
GOFF2
_
PEAK
is the peak inductor current (see
the Inductor Selection section). For ceramic capaci-
tors, the output-voltage ripple is typically dominated by
V
GOFF2
_
RIPPLE(C)
. The voltage rating and temperature
characteristics of the output capacitor must also be
considered. Note that all ceramic capacitors typically
have large temperature coefficient and bias voltage
coefficients. The actual capacitor value in the circuit is
typically significantly less than the stated value.
Input Capacitor Selection
The input capacitor reduces the current peaks drawn
from the input supply and reduces noise injection into
the IC. A 10FF ceramic capacitor is used in the typical
operating circuit (Figure 1) because of the high source
impedance seen in typical lab setups. Actual applica-
tions usually have much lower source impedance since
the step-up regulator often runs directly from the output
of another regulated supply. Typically, the input capaci-
tance can be reduced below the values used in the typi-
cal operating circuit.
Rectifier Diode
The MAX17122’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 1A Schottky diode
complements the internal MOSFET well.
Output-Voltage Selection
The output voltage of the step-up regulator is temperature
compensated. From the warm temperature range ((3.3V
- V
NTC
) > 1.65V), the output voltage is set by connecting
a resistive voltage-divider from the output (V
GOFF2
) to
the 3.3V reference with the center tap connected to FB3
(see Figure 1). Select R4 in the 10kI to 50kI range.
Calculate R3 with the following equation:
GOFF2_WARM FB3
FB3
V - V
R3 R4
V - 3.3V
= ×
where V
FB3
, the step-up regulator’s feedback set point,
is 1.65V. Place R3 and R4 close to the IC.
For cold temperatures ((3.3V - V
NTC
) < V
SET
), the output
voltage is set by:
GOFF2_COLD
SET
R4 V R3 3.3V
V
R3 R4
× + ×
=
+
If the above calculated V
SET
voltage is larger than
1.65V, then temperature compensation is disabled and
the boost-buck regulator output is V
GOFF2_WARM
at all
temperatures.
Calculate the SET pin resistor R
SET
as follows:
SET
SET
V
R
100 A
=
F
The temperature-compensation network is usually a
thermistor in series with a resistor as in Figure 1. A
parallel resistor is often added to linearize the network’s
resistance-temperature characteristic.
Loop Compensation
Choose R
COMP3
to set the high-frequency integrator
gain for fast-transient response. Choose C
COMP3
to set
the integrator zero to maintain loop stability. Typically, a
low bandwidth is expected for normal operation. In that
case, choosing C
COMP3
= 4.7nF and R
COMP3
between
1kI and 5kI gives a good combination of stability and
startup timing. Using greater than 4.7nF for C
COMP3
can cause an excessive startup delay due to the time
required to charge C
COMP3
.
Positive Charge-Pump Linear Regulators
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest number
of charge-pump stages that meet the output requirement.
The number of positive charge-pump stages is given by:
GON PNP AVDD
POS
AVDD D
V V - V
n
V - 2 V
+
=
×
where n
POS
is the number of positive charge-pump
stages, V
GON
is the output of the positive charge-pump