Datasheet

A good compromise between size and losses is typically
found at a 30% ripple current to load current ratio (LIR =
0.3), which corresponds to a peak inductor current 1.15
times the DC load current:
OUT IN OUT
IN SW LOAD(MAX)
V (V V )
L
V f I LIR
×−
=
×× ×
where I
LOAD(MAX)
is the maximum DC load current, and
the switching frequency fSW is 500kHz when FREQ is
tied to VL, and 250kHz when FREQ is tied to AGND. The
exact inductor value is not critical and can be adjusted to
make trade-offs among size, cost, and efficiency. Lower
inductor values minimize size and cost, but they also
increase the output ripple and reduce the efficiency due to
higher peak currents. On the other hand, higher inductor
values increase efficiency, but at some point increased
resistive losses due to extra turns of wire will exceed the
benefit gained from lower AC current levels.
The inductor’s saturation current must exceed the peak
inductor current. The peak current can be calculated by:
OUT IN OUT
RIPPLE
SW IN
RIPPLE
PEAK LOAD(MAX)
V (V V )
I
f LV
I
II
2
×−
=
××
= +
The inductor’s DC resistance should be low for good effi-
ciency. Find a low-loss inductor having the lowest possible
DC resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, though powdered iron is
inexpensive and can work well at 250kHz. Shielded-core
geometries help keep noise, EMI, and switching wave-
form jitter low.
MOSFET Selection and Current-Limit Setting
The MAX1530/MAX1531s’ step-down controller drives
two external logic-level N-channel MOSFETs. Since the
R
DS(ON)
of each MOSFET is used as a sense resistor to
provide current-sense signals to the PWM, their R
DS(ON)
values are important considerations in component selec-
tion.
The R
DS(ON)
of the high-side MOSFET (N1) provides an
inductor current-sense signal for current-mode operation
and also provides a crude maximum current limit during
the high-side on-time that prevents runaway currents if
the inductor saturates. The MOSFET voltage is measured
across the high-side MOSFET from VIN to LX and is lim-
ited to 400mV (typ). To ensure the desired output current
with sufficient margin, choose a MOSFET with R
DS(ON)
low enough that the peak current does not generate
more than 340mV across the MOSFET, even when the
MOSFET is hot. If the MOSFET’s R
DS(ON)
is not speci-
fied at a suitable temperature, use the maximum room
temperature specification and add 0.5% per °C for the
R
DS(ON)
increase with temperature:
PEAK DS(ON)_HOT
I R 340mV×<
To ensure stable operation of the current-mode PWM,
the minimum current-sense ripple signal should exceed
12mV. Since this value depends on the minimum R
DS(ON)
of the high-side MOSFET, which is not typically a speci-
fied parameter, a good rule of thumb is to choose the typi-
cal room temperature R
DS(ON)
about 2 times the amount
needed for this:
RIPPLE DS(ON)_TYP
I R 24mV×>
For example, Figure 6’s circuit is designed for 1.5A and
uses a dual MOSFET (N1) for both the high-side and low-
side MOSFETs. Its maximum R
DS(ON)
at room tempera-
ture is 145mΩ and an estimate of its maximum R
DS(ON)
at our chosen maximum temperature of +85°C is 188mΩ.
Since the inductor ripple current is 0.5A, the peak current
through the MOSFET is 1.75A. So the maximum peak
current-sense signal is 330mV, which is less than 340mV.
Using the typical R
DS(ON)
of 113mΩ and the ripple cur-
rent of 0.5A, the current ripple signal for the PWM is
56mV, much greater than the required 24mV.
The R
DS(ON)
of the low-side MOSFET (also N1) provides
current-limit information during the low-side ontime that
inhibits a high-side on-time if the MOSFET voltage is
too high. The voltage is measured across the low-side
MOSFET from PGND to LX and the threshold is set
by ILIM. To use the preset 250mV (typ) threshold, con-
nect ILIM to VL and choose a MOSFET with R
DS(ON)
low enough that the “valley” current does not generate
more than 190mV across the MOSFET, even when the
MOSFET is hot. If the MOSFET’s R
DS(ON)
is not speci-
fied at a suitable temperature, use the maximum room
temperature specification and add 0.5% per °C for the
R
DS(ON)
increase with temperature:
VALLEY OUT RIPPLE
VALLEY DS(ON)_HOT
I I I /2
I R 190mV
= −
×<
If the MOSFET’s R
DS(ON)
is lower than necessary, there
is no need to adjust the current-limit threshold using ILIM.
If the MOSFET’s R
DS(ON)
is too high, adjust the current-
limit threshold using a resistive-divider between VL and
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
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