Rainbow Electronics MAX1531 User Manual

Page 23

MAX1530/MAX1531

Multiple-Output Power-Supply Controllers for

LCD Monitors

______________________________________________________________________________________

load current ratio (LIR = 0.3), which corresponds to a
peak inductor current 1.15 times the DC load current:

where I

LOAD(MAX)

is the maximum DC load current,

and the switching frequency f

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 effi-
ciency. 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:

The inductor’s DC resistance should be low for good
efficiency. Find a low-loss inductor having the lowest
possible DC resistance that fits in the allotted dimen-
sions. 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 waveform 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 compo-

nent selection.

The R

DS(ON)

of the high-side MOSFET (N1) provides an

inductor current-sense signal for current-mode opera-
tion and also provides a crude maximum current limit
during the high-side on-time that prevents runaway cur-
rents if the inductor saturates. The MOSFET voltage is
measured across the high-side MOSFET from V

to LX

and is limited 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

DS(ON)

is not specified at a suitable temperature, use

the maximum room temperature specification and add
0.5% per °C for the R

DS(ON)

increase with temperature:

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 typical-

ly a specified parameter, a good rule of thumb is to
choose the typical room temperature R

DS(ON)

about 2

times the amount needed for this:

For example, Figure 1’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 tem-

perature is 145m

Ω and an estimate of its maximum

DS(ON)

at our chosen maximum temperature of +85°C

is 188m

Ω. Since the inductor ripple current is 0.5A, the

peak current through the MOSFET is 1.75A. So the maxi-
mum peak current-sense signal is 330mV, which is less
than 340mV. Using the typical R

DS(ON)

of 113m

Ω and

the ripple current 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) pro-

vides current-limit information during the low-side on-
time 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,
connect 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)

not specified at a suitable temperature, use the maxi-
mum room temperature specification and add 0.5% per
°C for the R

DS(ON)

increase with temperature:

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

VALLEY

OUT

RIPPLE

VALLEY

DS ON

HOT

−

(

) _

190

RIPPLE

DS ON

TYP

(

) _

PEAK

DS ON

HOT

(

) _

340

L V

RIPPLE

OUT

PEAK

LOAD MAX

RIPPLE

−

Ч Ч

(

)

(

)

LIR

OUT

LOAD MAX

−

(

)

(

)