Rainbow Electronics MAX1531 User Manual
Page 25
MAX1530/MAX1531
Multiple-Output Power-Supply Controllers for
LCD Monitors
______________________________________________________________________________________
25
amplifier g
m
, the compensation resistor R
COMP
, the FB
regulation V
FB
, and the output voltage set point V
OUT
:
Use the following to calculate the slope compensation
change during the sag:
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
capacitance and inductor slew rate is a function of the
load step, the output capacitor value, the inductor
value, and the output voltage:
The amplitude of the overshoot due to the AC load reg-
ulation is:
where
∆SC
OVER
is the change of the slope compensa-
tion during the overshoot, given by:
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 big-
ger 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
voltage 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 regu-
lates 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:
where V
FB
= 1.238V is the FB regulation voltage, A
VCS
is the gain of the current-sense amplifier (3.5 typical),
A
VEA
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:
R
LE
is the equivalent load resistance, given by:
In the above equation, D’ = 1 - D, n is a factor deter-
mined by the slope compensation m
c
and the inductor
current ramp m
1
, as shown below:
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:
m
V
V
L
R
A
IN
OUT
DS ON
VCS
1
(
)
=
Ч
Ч
-
n
m
m
C
= +
1
1
R
V
I
L
f
n
D D
LE
OUT
LOAD MAX
SW
=
Ч
Ч
'
(
)
||
-
A
V
R
A
V
R
A
DC
FB
LE
VEA
OUT SET
DS ON
VCS
=
Ч
Ч
Ч
Ч
(
)
(
)
I
V
V
V
A
V
R
A
PEAK
OUT
OUT SET
FB
VEA
OUT SET
DS ON
VCS
=
Ч
Ч
Ч
Ч
(
)
(
)
(
)
(
)
-
∆SC
mV
V
V
D
OVER
OUT
IN
OVER
=
×
.
437 5
-
V
V
A
R
I
SC
V
R
g
OVER AC
OUT
VCS
DS ON
LOAD
OVER
FB
COMP
m
_
(
)
=
Ч
Ч
Ч
+
Ч
Ч
∆
∆
V
L
I
C
V
SOAR LC
LOAD
OUT
OUT
_
(
)
=
Ч
Ч
Ч
∆
2
2
∆SC
mV
D
V
V
UNDER
UNDER
OUT
IN
=
×
.
437 5
-
V
V
A
R
I
SC
V
R
g
UNDER AC
OUT
VCS
DS ON
LOAD
UNDER
FB
COMP
m
_
(
)
=
Ч
Ч
Ч
+
Ч
Ч
∆
∆