Datasheet

MAX1533A/MAX1537A
High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
______________________________________________________________________________________ 23
Output Voltage
DC output accuracy specifications in the Electrical
Characteristics table refer to the error-comparator’s
threshold. When the inductor continuously conducts,
the MAX1533A/MAX1537A regulate the peak of the out-
put ripple, so the actual DC output voltage is lower than
the slope-compensated trip level by 50% of the output
ripple voltage. For PWM operation (continuous conduc-
tion), the output voltage is accurately defined by the fol-
lowing equation:
where V
NOM
is the nominal output voltage, A
SLOPE
equals 1%, and V
RIPPLE
is the output ripple voltage
(V
RIPPLE
= ESR x ΔI
INDUCTOR
as described in the
Output Capacitor Selection section).
In discontinuous conduction (I
OUT
< I
LOAD(SKIP)
), the
MAX1533A/MAX1537A regulate the valley of the output
ripple, so the output voltage has a DC regulation level
higher than the error-comparator threshold. For PFM
operation (discontinuous conduction), the output volt-
age is approximately defined by the following equation:
where V
NOM
is the nominal output voltage, f
OSC
is the
maximum switching frequency set by the internal oscil-
lator, f
SW
is the actual switching frequency, and I
IDLE
is
the idle-mode inductor current when pulse skipping.
Adjustable/Fixed Output Voltages
(Dual-Mode Feedback)
Connect FB3 and FB5 to GND to enable the fixed
SMPS output voltages (3.3V and 5V, respectively), set
by a preset, internal resistive voltage-divider connected
between CSL_ and analog ground. Connect a resistive
voltage-divider at FB_ between CSL_ and GND to
adjust the respective output voltage between 1V and
5.5V (Figure 5). Choose R2 (resistance from FB to
GND) to be about 10kΩ and solve for R1 (resistance
from OUT to FB) using the equation:
where V
FB_
= 1V nominal.
RR
V
V
OUT
FB
12 1
_
_
=
⎛
⎝
⎜
⎞
⎠
⎟
−
VV
f
f
I ESR
OUT PFM NOM
SW
OSC
IDLE()
=+
⎛
⎝
⎜
⎞
⎠
⎟
×
1
2
VV
AV
V
V
OUT PWM NOM
SLOPE NOM
IN
RIPPLE
()
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
1
2
- -
t
ON(SKIP)
≥
I
IDLE
L
V
IN
- V
OUT
INDUCTOR CURRENT
I
LOAD(SKIP)
TIME
ON-TIME
0
CSL
TO ERROR
AMPLIFIER
REF
(2.0V)
R
12R
FB
FIXED OUTPUT
FB = GND
ADJUSTABLE
OUTPUT
Figure 4. Pulse-Skipping/Discontinuous Crossover Point Figure 5. Dual-Mode Feedback Decoder