MAX8728ETJ+ Maxim Integrated Products, MAX8728ETJ+ Datasheet - Page 23

• Manufacturer Part Number: MAX8728ETJ+
• Description: Display Drivers Low-Cost Multiple-Ou tput Power Supply fo
• Manufacturer: Maxim Integrated Products

Specifications of MAX8728ETJ+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Since the MAX8728’s step-down regulator is internally
compensated, it is stable with any reasonable amount
of output capacitance. However, the actual capaci-
tance 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: varia-
tions 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:
where I
Regulator Design, Inductor Selection section, C
output capacitance, and R
put capacitor C
ripple current is 0.6A. If the voltage ripple requirement
of Figure 1’s circuit is ±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 55mΩ
and the output capacitance should be more than 1.5µF
to meet the total ripple requirement. A 22µF capacitor
with ESR (including PC board trace resistance) of 10mΩ
is selected for the standard application circuit in Figure 1,
which easily meets the voltage-ripple requirement.
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 formulae 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:
The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor
V
OUT RIPPLE
V
OUT RIPPLE ESR
1
V
_
OUT1
OUT1_ESR_STEP
1
V
_
OUT RIPPLE C
_
1
_
RIPPLE
OUT1
=
(
______________________________________________________________________________________
V
OUT RIPPLE ESR
. In Figure 1’s circuit, the inductor
)
( )
1
=
is defined in the Step-Down
_
= ∆I
I
OUT RIPPLE
=
Output-Capacitor Selection
ESR
8
OUT1
1
Power Supply for LCD Monitors/TVs
_
×
_
(
I
OUT RIPPLE
OUT1
C
OUT
)
x R
1
_
+
is the ESR of out-
1
ESR_OUT1
×
V
×
OUT RIPPLE C
R
ESR OUT
f
SW
1
_
_
OUT1
1
( )
Low-Cost, Multiple-Output
is
value, the input-to-output voltage differential, and the
maximum duty cycle:
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor
value and the output voltage:
Given the component values in the circuit of Figure 1,
during a 2A step-load transient, the voltage step due to
capacitor ESR is negligible. The voltage sag and soar
are 40.2mV and 71.6mV, respectively.
The MAX8728’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 2A Schottky
diode works well in the MAX8728’s step-down regulator.
Connect a resistive voltage-divider between OUT1 and
GND with the center tap connected to FB1 to adjust the
output voltage. Choose R12 (resistance from FB1 to
GND) to be between 5kΩ and 50kΩ, and solve for R11
(resistance from OUT1 to FB1) using the equation:
where V
Connecting a small capacitor (e.g., 47pF) between FB1
and GND reduces FB1 noise sensitivity.
The inductance value, peak-current rating, and series
resistance are factors to consider when selecting the
step-up inductor. These factors influence the convert-
er’s efficiency, maximum output load capability, tran-
sient response time, and output voltage ripple. Physical
size and cost are also important factors to be consid-
ered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
V
OUT SAG
1
_
V
FB1
OUT SOAR
1
= 2V, and V
=
R
_
11
2
×
=
C
R
OUT
=
12
Step-Up Regulator Design
OUT1
1
2
L
×
L
×
OUT
×
OUT



(
Output-Voltage Selection
V
C
V
IN MIN
may vary from 2V to 3.6V.
1
V
1
OUT
OUT
(
FB
×
×
1
(
1
∆
)
1
(
∆
Inductor Selection
I
OUT
×
−
×
I
OUT
1
D



1
Rectifier Diode
V
MAX
)
OUT
2
1
)
2
1
−
V
OUT
1
)
23