Datasheet

MAX1533A/MAX1537A
High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
______________________________________________________________________________________ 29
The transformer turns ratio (N) is determined by:
where V
SEC
is the minimum required rectified sec-
ondary voltage, V
FWD
is the forward drop across the
secondary rectifier, V
OUT5(MIN)
is the minimum value of
the main output voltage, and V
RECT
is the on-state volt-
age drop across the synchronous-rectifier MOSFET.
The transformer secondary return is often connected to
the main output voltage instead of ground to reduce
the necessary turns ratio. In this case, subtract V
OUT5
from the secondary voltage (V
SEC
- V
OUT5
) in the trans-
former turns-ratio equation above. The secondary
diode in coupled-inductor applications must withstand
flyback voltages greater than 60V. Common silicon rec-
tifiers, such as the 1N4001, are also prohibited
because they are too slow. Fast silicon rectifiers such
as the MURS120 are the only choice. The flyback volt-
age across the rectifier is related to the V
IN
- V
OUT5
dif-
ference, according to the transformer turns ratio:
V
FLYBACK
= V
SEC
+ (V
IN
- V
OUT5
) x N
where N is the transformer turns ratio (secondary wind-
ings/primary windings), and V
SEC
is the maximum sec-
ondary DC output voltage. If the secondary winding is
returned to V
OUT5
instead of ground, subtract V
OUT5
from V
FLYBACK
in the equation above. The diode’s
reverse-breakdown voltage rating must also accommo-
date any ringing due to leakage inductance. The
diode’s current rating should be at least twice the DC
load current on the secondary output.
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V
IN
- V
OUT
dif-
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The total output voltage sag is the sum of the voltage
sag while the inductor is ramping up, and the voltage
sag before the next pulse can occur.
where D
MAX
is the maximum duty factor (see the
Electrical Characteristics table), T is the switching period
(1/f
OSC
), and ΔT equals V
OUT
/V
IN
x T when in PWM
mode, or L x 0.2 x I
MAX
/(V
IN
- V
OUT
) when in skip mode.
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculat-
ed as:
Setting the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The
peak inductor current occurs at I
LOAD(MAX)
plus half
the ripple current; therefore:
where I
LIMIT
equals the minimum current-limit threshold
voltage divided by the current-sense resistance
(R
SENSE
). For the default setting, the minimum current-
limit threshold is 70mV.
Connect ILIM_ to V
CC
for the default current-limit
threshold. In adjustable mode, the current-limit thresh-
old is precisely 1/10th the voltage seen at ILIM_. For an
adjustable threshold, connect a resistive divider from
REF to analog ground (GND) with ILIM_ connected to
the center tap. The external 500mV to 2V adjustment
range corresponds to a 50mV to 200mV current-limit
threshold. When adjusting the current limit, use 1% tol-
erance resistors and a divider current of approximately
10µA to prevent significant inaccuracy in the current-
limit tolerance.
The current-sense method (Figure 9) and magnitude
determine the achievable current-limit accuracy and
power loss. Typically, higher current-sense limits pro-
vide tighter accuracy, but also dissipate more power.
Most applications employ a current-limit threshold
(V
LIMIT
) of 50mV to 100mV, so the sense resistor can
be determined by:
R
SENSE
= V
LIMIT
/I
LIM
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure
II
I
LIMIT LOAD MAX
INDUCTOR
>+
⎛
⎝
⎜
⎞
⎠
⎟
()
Δ
2
V
IL
CV
SOAR
LOAD MAX
OUT OUT
=
()
()
Δ
2
2
V
LI
CVD V
ITT
C
SAG
LOAD MAX
OUT IN MAX OUT
LOAD MAX
OUT
=
()
×
()
+
()
()
()
Δ
ΔΔ
2
2 -
-
N
VV
VVV
SEC FWD
OUT RECT SENSE
=
+
++
5