Datasheet

AGND at ILIM. The threshold is approximately 1/5th the
voltage on ILIM over a range of 0.25V to 3V:
VALLEY DS(ON)_HOT ILIM
I R 0.2 V (1 K )× < × ×−
K is the accuracy of the current-limit threshold, which is
20% when the threshold is 250mV.
For example, Figure 1’s N1 MOSFET has a maximum
R
DS(ON)
at room temperature of 145mΩ and an estimate
of its maximum at our chosen maximum temperature of
+85°C is 188mΩ. Since the inductor ripple current is 0.5A,
the valley current through the MOSFET is 1.25A. So the
maximum valley current-sense signal is 235mV, which is
too high to work with the 190mV minimum of the default
current-limit threshold. Adding a divider at ILIM (R12 and
R13) adjusts the ILIM voltage to 1.7V and the current-limit
threshold to 340mV, providing more than adequate mar-
gin for threshold accuracy.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduce noise and voltage
ripple on the input caused by the regulator’s switching.
It is 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 IN OUT
RMS LOAD
IN
V (V V )
II
V
×−
= ×
The worst case is I
RMS
= 0.5 × I
LOAD
, which occurs at
V
IN
= 2 × V
OUT
.
For most applications, ceramic capacitors are used
because of their high ripple current and surge current
capabilities. For 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.
Output Capacitor
The output capacitor and its equivalent series resistance
(ESR) affect the regulator’s loop stability, output ripple volt-
age, and transient response. The Compensation Design
section discusses the output capacitance requirement
based on the loop stability. This section deals with how to
determine the output capacitance and ESR needs accord-
ing 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 volt-
age drop across the capacitor’s ESR caused by the cur-
rent into and out of the capacitor:
RIPPLE RIPPLE(ESR) RIPPLE(C)
RIPPLE(ESR) RIPPLE ESR
RIPPLE
RIPPLE(C)
OUT SW
VV V
V IR
I
V
8C f
= +
= ×
=
××
where C
OUT
is the output capacitance, and R
ESR
is the
ESR of the output capacitor. In Figure 1’s circuit, the
inductor ripple current is 0.5A. Assume the voltage-ripple
requirement is 2% (peak-to-peak) of the 3.3V output,
which corresponds to 66mV total peak-to-peak ripple.
Assuming that the ESR ripple component and the capaci-
tive ripple component each should be less than 50% of
the 66mV total peak-to-peak ripple, then the ESR should
be less than 66mΩ and the output capacitance should
be more than 7.6μF to meet the total ripple requirement.
A 22μF ceramic capacitor with ESR (including PC board
trace resistance) of 10mΩ is selected for the standard
application circuit in Figure 1, which easily meets the volt-
age ripple requirement.
The step-down regulator’s output capacitance and ESR
also affect the voltage undershoot and overshoot when
the load steps up and down abruptly. The undershoot
and overshoot have three components: the voltage steps
caused by ESR, the voltage undershoot and overshoot
due to the current-mode control’s AC load regulation, and
the voltage sag and soar due to the finite capacitance and
inductor slew rate.
The amplitude of the ESR steps is a function of the load
step and the ESR of the output capacitor:
ESR_STEP LOAD ESR
V IR=∆×
The amplitude of the sag due to the finite output capaci-
tance and inductor slew rate 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
LOAD
SAG_LC
OUT IN(MIN) MAX OUT
L (I )
V
2 C (V D -V )
×∆
=
×× ×
The amplitude of the undershoot due to the AC load regu-
lation is a function of the high-side MOSFET R
DS(ON)
, the
gain of the current-sense amplifier A
VCS
, the change of the
slope compensation during the undershoot (ΔSC
UNDER
),
the transconductance of the error amplifier g
m
, the com-
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
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