Datasheet

pensation resistor R
COMP
, the FB regulation V
FB
, and
the output voltage set point V
OUT
:
VCS DS(ON) LOAD
OUT
UNDER
UNDER_AC
FB COMP m
AR I
V
SC
V
VR g
× ×∆

×


+∆

=
××
Use the following to calculate the slope compensation
change during the sag:
OUT
UNDER UNDER
IN
V
SC 437.5mV D -
V

∆=×


where D
UNDER
is the duty cycle at the valley of the sag,
which is usually 50%.
The actual undershoot is always equal to or bigger than
the worst of V
ESR_STEP
, V
SAG_LC
, and V
UNDER_AC
.
The amplitude of the soar 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, and the
output voltage:
2
LOAD
SOAR_LC
OUT OUT
L (I )
V
2C V
×∆
=
××
The amplitude of the overshoot due to the AC load regu-
lation is:
VCS DS(ON) LOAD
OUT
OVER
OVER_AC
FB COMP m
AR I
V
SC
V
VR g
× ×∆

×


+∆

=
××
where ΔSC
OVER
is the change of the slope compensa-
tion during the overshoot, given by:
OUT
OVER OVER
IN
V
SC 437.5mV - D
V

∆=×


where D
OVER
is the duty cycle at the peak of the over-
shoot, which is typically 0%.
Similarly, the actual overshoot is always equal to or
bigger than the worst of V
ESR_STEP
, V
SOAR_LC
, and
V
OVER_AC
.
Given the component values in the circuit of Figure 1,
during a 1.5A step load transient, the voltage step due to
capacitor ESR is negligible. The voltage sag due to finite
capacitance and inductor slew rate is 81mV, and the volt-
age undershoot due to the AC load regulation is 170mV.
The total undershoot seen in the Typical Operating
Characteristics is 170mV. The voltage soar due to finite
capacitance and inductor slew rate is 155mV, and the
voltage overshoot due to the AC load regulation is 167mV.
The total overshoot seen the in the Typical Operating
Characteristics is 200mV.
Compensation Design
The step-down controller of the MAX1530/MAX1531 uses
a peak current-mode control scheme that regulates the
output voltage by forcing the required current through the
inductor. The MAX1530/MAX1531 use the voltage across
the high-side MOSFET’s R
DS(ON)
to sense the inductor
current. Using the current-sense amplifier’s output signal
and the amplified feedback voltage sensed at FB, the
control loop sets the peak inductor current by:
OUT OUT(SET) FB VEA
PEAK
OUT(SET) DS(ON) VCS
(V -V ) V A
I
V RA
××
=
××
where V
FB
= 1.238V is the FB regulation voltage, A
VCS
is
the gain of the current-sense amplifier (3.5 typical), AVEA
is the DC gain of the error amplifier (2000 typ), V
OUT(SET)
is the output voltage set point, and R
DS(ON)
is the on-
resistance of the high-side MOSFET.
The total DC loop gain (A
DC
) is approximately:
FB LE VEA
DC
OUT(SET) DS(ON) VCS
V R A
A
V RA
××
=
××
RLE is the equivalent load resistance, given by:
OUT SW
LE
LOAD(MAX)
V L f
R
I n D'- D
||

×


=


×


In the above equation, D’ = 1 - D, n is a factor determined-
by the slope compensation mc and the inductor current
ramp m1, as shown below:
C
1
m
n 1
m
= +
The slope compensation of the MAX1530/MAX1531 is
219mV/μs. The inductor current ramp is a function of
the input voltage, output voltage, inductance, high-side
MOSFET on-resistance R
DS(ON)
, and the gain of the
current-sense amplifier A
VCS
, and is:
IN OUT
1 DS(ON) VCS
V -V
m R A
L
= ××
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
Maxim Integrated │ 25