Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
28 _____________________________________________________________________________________
Using the circuit’s minimum input voltage under normal
operation (12V) and estimating efficiency of 85% at that
operating point:
IN(DC,MAX)
2.2A 15V
I 3.235A
12V 85%
×
= ≈
×
The ripple current and the peak current are:
( )
AVDD_RIPPLE
12V 15V -12V
I 0.68A
4.7 H 15V 750kHz
×
= ≈
× ×F
AVDD_PEAK
0.68A
I 3.235A 3.575A
2
= + ≈
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:
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 capaci-
tors, 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 con-
sidered. Note that all ceramic capacitors typically have
large temperature coefficient and bias voltage coef-
ficients. 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 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 3A 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 AGND 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.
HVS Function
When HVS exceeds its logic-high threshold, R
HVS
con-
nects to AGND, effectively placing R
HVS
in parallel with
the low-side resistor-divider (R2) and regulates V
AVDD
to
a higher voltage V
AVDD(HIGH)
. Connect the HVS pin to
ground to disable this function. Calculate R
HVS
with the
following equation:
HVS
AVDD(HIGH)
FB1
R1 R2
R
V
R2 -1 - R1
V
×
=
 
 
 
Loop Compensation
Choose R
COMP1
to set the high-frequency integrator
gain for fast-transient response. Choose C
COMP1
to set
the integrator zero to maintain loop stability. Add a small
capacitor (C
P1
) from COMP1 to AGND to reduce jitter
and improve stability. Usually 10pF is enough for this
purpose.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
IN AVDD AVDD
COMP1
AVDD AVDD(MAX)
100 V V C
R
L I
× × ×
≈
×
AVDD AVDD
COMP1
AVDD(MAX) COMP
V C
C
10 I R
×
≈
× ×
To further optimize transient response, vary R
COMP1
in
20% steps and C
COMP1
in 50% steps while observing
transient-response waveforms.