MAX8723ETE+T Maxim Integrated Products, MAX8723ETE+T Datasheet - Page 14

• Manufacturer Part Number: MAX8723ETE+T
• Description: IC REG FOR LCD DISPLAY 16-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8723ETE+T

• Applications: Converter, LCD
• Voltage - Input: 6 ~ 13.2 V
• Number Of Outputs: 1
• Voltage - Output: 3.3V, 2 ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 cur-
rent requirements and voltage rating, rather than
capacitance value. The input voltage and load current
determine the RMS input ripple current (I
The worst case is I
V
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 +10°C
temperature rise at the RMS input current corresponding
to the maximum load current. If necessary, add multiple
capacitors in parallel to distribute the ripple current.
Since the MAX8723’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
section, C
R
Figure 1’s circuit, the inductor peak-to-peak ripple cur-
rent 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
Low-Cost, Internal-Switch, Step-Down
Regulator for LCD Displays
14
IN
ESR
V
OUT RIPPLE
= 2 x V
______________________________________________________________________________________
V
_
OUT RIPPLE ESR
OUT
_
OUT
I
_
RMS
OUT
V
is the ESR of the output capacitor C
_
OUT RIPPLE C
OUT
RIPPLE
.
=
=
_
I
OUT
V
(
is the output capacitance, and
OUT RIPPLE ESR
RMS
is defined in the Inductor Selection
Output Capacitor Selection
x
)
_
=
( )
= 0.5 x I
V
I
OUT RIPPLE
OUT
=
8
_
(
×
×
C
(
V
V
IN
OUT
OUT
Input Capacitors
I
IN
)
OUT
+
−
, which occurs at
V
×
V
×
OUT RIPPLE C
OUT
R
f
RMS
SW
ESR OUT
_
)
):
_
OUT
( )
. In
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
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 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:
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:
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 1.5MHz 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 MAX8723’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. Based on the 2A capability of
the high-side switch, a 2A Schottky diode works well for
most applications. The Schottky diode’s current rating
can be reduced or the diode can be omitted entirely, if
an external synchronous rectifier MOSFET is used.
V
OUT SAG
meet
_
V
OUT_ESR_STEP
V
OUT SOAR
the
=
2
_
×
C
OUT
total
=
= ∆I
L
×
L
2
OUT
(
OUT
×
V
OUT
IN MIN
C
ripple
(
OUT
×
×
(
x R
(
∆
∆
)
I
×
Rectifier Diode
OUT
I
×
OUT
ESR_OUT
V
D
OUT
MAX
requirement.
)
)
2
2
−
V
OUT
)