MAX1540A Datasheet, PDF(28/49 Page) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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MAX1540A Datasheet, PDF (28/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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Dual Step-Down Controllers with Saturation

Protection, Dynamic Output, and Linear Regulator

Inductor-Saturation Limit

The LSAT connection selects an upper current-sense

limit as the inductor-saturation threshold, or disables

the inductor-saturation protection feature altogether

(LSAT = GND). When enabled, the inductor-saturation

threshold is a multiple of the positive valley current-limit

threshold (Table 5) and tracks the valley current limit

when ILIM is adjusted. The selected inductor-saturation

threshold should give sufficient headroom above the

peak inductor current so switching noise does not acci-

dentally trip the saturation protection. Selecting an

excessively high threshold may allow inductor satura-

tion to go undetected. For an inductor with a low LIR

(the ratio of the inductor ripple current to the designed

maximum load current) near 20%, select the lowest sat-

uration threshold of 1.5 x ILIM(VAL) (LSAT = REF). When

using an inductor with a higher LIR, increase the induc-

tor-saturation threshold accordingly.

When inductor-saturation protection is enabled, the

MAX1540A/MAX1541 continuously monitor the inductor

current through the voltage across the current-sense

resistor. When the inductor-saturation threshold is

exceeded, the MAX1540A/MAX1541 immediately turn

off the high-side gate driver and enable a 6ÂµA dis-

charge current on ILIM_ (Figure 7) at the beginning of

the next DH_ on-time. This reduces the voltage on

ILIM_ by ÎVILIM where:

ÎVILIM

-ââ

RARB

RA +RB

â âIILIM(LSAT)

MAX1540A

MAX1541

REF

TO VALLEY

CURRENT-LIMIT

ILIM

COMPARATOR

(FIGURE 2)

6Î¼A

FROM LSAT

COMPARATOR

AND LOGIC

(FIGURE 2)

CREF

CILIM

where the ILIM saturation fault sink current (IILIM(LSAT))

is typically 6ÂµA (see the Electrical Characteristics

table). When using the default 50mV valley current-limit

threshold (ILIM_ = VCC), the ILIM_ saturation fault sink

current does not lower the current-limit threshold

(Figure 5).

If the inductor current remains below the saturation

threshold during the next cycle, the controller disables

the ILIM_ discharge current, allowing the ILIM_ voltage

to return to its nominal set point. The inductor should

not remain in saturation once the controller reduces the

valley current limit. If the inductor remains saturated,

the output voltage may drop low enough to trip the

undervoltage fault protection (UVP enabled), causing

the MAX1540A/MAX1541 to set the fault latch and shut

down both outputs. Adding a capacitor from ILIM_ to

GND slows the ILIM_ voltage change by the time con-

stant Ï = (RA || RB) x CILIM, where Ï is between 5 to 10

switching periods. If the inductor saturation occurs only

during a short load transient, the time constant allows

the power supply to recover before the output voltage

drops below the output undervoltage threshold.

Set ÎVILIM to be at least 30% of the ILIM_ set voltage.

Calculate RA and RB using the equations below:

VREF

IILIM(LSAT)

ÎVILIM

VILIM(SET)

with

ÎVILIM

VILIM(SET)

set

30%

ââ

VREF

VILIM(SET)

-1â â

Inductor-saturation sensing works best when using a cur-

rent-sense resistor in series with the inductor. See the

Setting the Current Limit section for various current-sense

configurations (Figure 14) and LSAT recommendations.

Table 5. LSAT Configuration Table

LSAT

VCC

Open

REF

GND

INDUCTOR-SATURATION THRESHOLD

2.00 x ILIM(VAL)

1.75 x ILIM(VAL)

1.50 x ILIM(VAL)

Disabled

Figure 7. Adjustable Current-Limit Threshold

28 ______________________________________________________________________________________

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