Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
______________________________________________________________________________________ 29
The step-down regulator’s output capacitor and ESR
also affect the voltage undershoot and overshoot when
the load steps up and down abruptly. The undershoot
and overshoot also have two components: the voltage
steps caused by ESR, and voltage sag and soar due to
the finite capacitance and inductor slew rate. Use the
following formulas to check if the ESR is low enough
and the output capacitance is large enough to prevent
excessive soar and sag.
The amplitude of the ESR step is a function of the load
step and the ESR of the output capacitor:
OUT_ESR_STEP OUT ESR_OUT
V I R= ×D
The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor value,
the input-to-output voltage differential, and the maximum
duty cycle:
( )
2
2 OUT
OUT_SAG
OUT IN2(MIN) MAX OUT
L ( I )
V
2 C V D - V
×
=
× × ×
D
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor value,
and the output voltage:
2
2 OUT
OUT_SOAR
OUT OUT
L ( I )
V
2 C V
×
=
× ×
D
Keeping the full-load overshoot and undershoot less than
3% ensures that the step-down regulator’s natural integrator
response dominates. Given the component values in the
circuit of Figure 1, during a full 1.5A step load transient, the
voltage step due to capacitor ESR is negligible. The voltage
sag and soar are 76mV and 73mV, respectively.
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 works well in the MAX17126/MAX17126A’s
step-up regulator.
Step-Up Regulator
Inductor Selection
The inductance value, peak current rating, and series
resistance are factors to consider when selecting the
inductor. These factors influence the converter’s efficiency,
maximum output load capability, transient response time,
and output voltage ripple. Physical size and cost are also
important factors to be considered.
The maximum output current, input voltage, output
voltage, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple, and therefore, reduce the peak current, which
decreases core losses in the inductor and I
2
R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of wire
that increase physical size and can increase I
2
R losses
in the inductor. Low inductance values decrease the
physical size, but increase the current ripple and peak
current. Finding the best inductor involves choosing the
best compromise between circuit efficiency, inductor
size, and cost.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full-load current.
The best trade-off between inductor size and circuit
efficiency for step-up regulators generally has an LIR
between 0.3 and 0.5. However, depending on the AC
characteristics of the inductor core material and ratio of
inductor resistance to other power-path resistances, the
best LIR can shift up or down. If the inductor resistance
is relatively high, more ripple can be accepted to
reduce the number of turns required and increase the
wire diameter. If the inductor resistance is relatively low,
increasing inductance to lower the peak current can
decrease losses throughout the power path. If extremely
thin high-resistance inductors are used, as is common
for LCD panel applications, the best LIR can increase to
between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions.
Calculate the approximate inductor value using the
typical input voltage (V
IN
), the maximum output current
(I
AVDD(MAX)
), the expected efficiency (E
TYP
) taken
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
IN AVDD IN TYP
1
AVDD AVDD(MAX) SW
V V - V
L
V I f LIR
 
 
η
 
=
 
 
 
 
×
 
 
 
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V
IN(MIN)
using conservation
of energy and the expected efficiency at that operating
point (E
MIN
) taken from an appropriate curve in the
Typical Operating Characteristics:
AVDD(MAX) AVDD
IN(DC,MAX)
IN(MIN) MIN
I V
I
V
×
=
× η