Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
28 _____________________________________________________________________________________
where I
OUT(MAX)
is the maximum DC load current, and
the switching frequency f
SW
is 750kHz when FSEL is
tied to VL, 500kHz when FSEL is tied to GND. The exact
inductor value is not critical and can be adjusted to
make trade-offs among size, cost, and efficiency. Lower
inductor values minimize size and cost, but they also
increase the output ripple and reduce the efficiency
due to higher peak currents. On the other hand, higher
inductor values increase efficiency, but at some point
resistive losses due to extra turns of wire exceed the
benefit gained from lower AC current levels.
The inductor’s saturation current must exceed the peak
inductor current. The peak current can be calculated by:
( )
OUT IN2 OUT
OUT_RIPPLE
SW 2 IN2
V V - V
I
f L V
×
=
× ×
OUT_RIPPLE
OUT_PEAK OUT(MAX)
I
I I
2
= +
The inductor’s DC resistance should be low for good
efficiency. Find a low-loss inductor having the lowest
possible DC resistance that fits in the allotted dimensions.
Ferrite cores are often the best choice. Shielded-
core geometries help keep noise, EMI, and switching
waveform jitter low.
Considering the typical operation circuit in Figure 1, the
maximum load current I
OUT(MAX)
is 1.5A with a 3.3V
output and a typical 12V input voltage. Choosing an LIR
of 0.4 at this operation point:
2
3.3V (12V - 3.3V)
L 5.3 H
12V 750kHz 1.5A 0.4
×
= ≈
× × ×
F
Pick L
2
= 4.7FH. At that operation point, the ripple current
and the peak current are:
( )
OUT_RIPPLE
3.3V 12V - 3.3V
I 0.68A
750kHz 4.7 H 12V
×
= =
× ×F
OUT_PEAK
0.68A
I 1.5A 1.84A
2
= + =
Input Capacitors
The input filter capacitors reduce peak currents drawn
from the power source and reduce noise and voltage
ripple on the input caused by the regulator’s switching.
They are usually selected according to input ripple
current requirements and voltage rating, rather than
capacitance value. The input voltage and load current
determine the RMS input ripple current (I
RMS
):
( )
OUT IN2 OUT
RMS OUT
IN2
V V - V
I I
V
×
= ×
The worst case is I
RMS
= 0.5 x I
OUT
that occurs at V
IN2
= 2 x V
OUT
.
For most applications, ceramic capacitors are used
because of their high ripple current and surge current
capabilities. For optimal circuit long-term reliability,
choose an input capacitor that exhibits less than +10NC
temperature rise at the RMS input current corresponding
to the maximum load current.
Output Capacitor Selection
Since the MAX17126/MAX17126As’ step-down regulator
is internally compensated, it is stable with any reasonable
amount of output capacitance. However, the actual
capacitance and equivalent series resistance (ESR)
affect the regulator’s output ripple voltage and transient
response. The rest of this section deals with how to
determine the output capacitance and ESR needs
according to the ripple voltage and load-transient
requirements.
The output voltage ripple has two components: variations
in the charge stored in the output capacitor, and the
voltage drop across the capacitor’s ESR caused by the
current into and out of the capacitor:
OUT_RIPPLE OUT_RIPPLE(ESR) OUT_RIPPLE(C)
V V V= +
OUT_RIPPLE(ESR) OUT_RIPPLE ESR_OUT
V I R= ×
OUT_RIPPLE
OUT_RIPPLE(C)
OUT SW
I
V
8 C f
=
× ×
where I
OUT
_
RIPPLE
is defined in the Step-Down Regulator
Inductor Selection section, C
OUT
(C5 in Figure 1) is the
output capacitance, and R
ESR
_
OUT
is the ESR of the
output capacitor C
OUT
. In Figure 1’s circuit, the inductor
ripple current is 0.68A. If the voltage-ripple requirement of
Figure 1’s circuit is P 1% of the 3.3V output, then the total
peak-to-peak ripple voltage should be less than 66mV.
Assuming that the ESR ripple and the capacitive ripple
each should be less than 50% of the total peak-to-peak
ripple, then the ESR should be less than 48.5mI and the
output capacitance should be more than 3.4FF to meet
the total ripple requirement. A 22FF capacitor with ESR
(including PCB trace resistance) of 10mI is selected
for the typical operating circuit in Figure 1, which easily
meets the voltage ripple requirement.