Datasheet
Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
30 _____________________________________________________________________________________
Calculate the ripple current at that operating point and
the peak current required for the inductor:
( )
IN(MIN) AVDD IN(MIN)
AVDD_RIPPLE
AVDD AVDD SW
V V - V
I
L V f
×
=
× ×
AVDD_RIPPLE
AVDD_PEAK IN(DC,MAX)
I
I I
2
= +
The inductor’s saturation current rating and the MAX17126/
MAX17126As’ LX1 current limit should exceed I
AVDD
_
PEAK
and the inductor’s DC current rating should exceed
I
IN(DC,MAX)
. For good efficiency, choose an inductor with
less than 0.1I series resistance.
Considering the typical operating circuit (Figure 1), the
maximum load current (I
AVDD(MAX)
) is 1A with a 16V
output and a typical input voltage of 12V. Choosing
an LIR of 0.3 and estimating efficiency of 90% at this
operating point:
2
1
12V 16V - 12V 90%
L 9 H
16V 1A 750kHz 0.3
= =
×
F
Using the circuit’s minimum input voltage (8V) and
estimating efficiency of 85% at that operating point:
IN(DC,MAX)
1A 16V
I 2.35A
8V 85%
×
= ≈
×
The ripple current and the peak current are:
( )
AVDD_RIPPLE
8V 16V - 8V
I 0.53A
10 H 16V 750kHz
×
= ≈
× ×F
AVDD_PEAK
0.53A
I 2.35A 2.62A
2
= + ≈
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
of the output capacitance, and the ohmic ripple due to
the capacitor’s equivalent series resistance (ESR):
AVDD_RIPPLE AVDD_RIPPLE(C) AVDD_RIPPLE(ESR)
V V V= +
AVDD AVDD IN
AVDD_RIPPLE(C)
AVDD AVDD SW
I V - V
V
C V f
≈
and:
AVDD_RIPPLE(ESR) AVDD_PEAK ESR_AVDD
V I R≈
where I
AVDD
_
PEAK
is the peak inductor current (see
the Inductor Selection section). For ceramic capacitors,
the output voltage ripple is typically dominated by
V
AVDD
_
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 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 22FF ceramic capacitor is used in the
typical operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source impedance
since the step-up regulator often runs directly from the
output of another regulated supply. Typically, the input
capacitance can be reduced below the values used in
the typical operating circuit.
Rectifier Diode
The MAX17126/MAX17126As’ 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 2A
Schottky diode complements the internal MOSFET well.
Output Voltage Selection
The output voltage of the step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V
AVDD
) to GND with the center tap connected
to FB1 (see Figure 1). Select R2 in the 10kI to 50kI
range. Calculate R1 with the following equation:
AVDD
FB1
V
R1 R2 -1
V
= ×
where V
FB1
, the step-up regulator’s feedback set point,
is 1.25V. Place R1 and R2 close to the IC.
Loop Compensation
Choose R
COMP
to set the high-frequency integrator gain
for fast-transient response. Choose C
COMP
to set the
integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following
equations to obtain stable performance and good
transient response:
IN AVDD AVDD
COMP
AVDD AVDD(MAX)
100 V V C
R
L I
× × ×
≈
×
AVDD AVDD
COMP
AVDD(MAX) COMP
V C
C
10 I R
×
≈
× ×