MAX1540A Datasheet, PDF(33/49 Page) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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MAX1540A Datasheet, PDF (33/49 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

LX_

MAX1540A/MAX1541

DL_

GND

RSENSE

COUT

CSP_

CSN_

OUT_

FB_

Figure 11. Setting VOUT with a Resistive Voltage-Divider at FB_

light loads, quickly discharging the output capacitors. If

fault blanking is enabled, the MAX1541 also disables

the main controllerâs (OUT1) overvoltage and undervolt-

age fault protection, and forces PGOOD1 to a high-

impedance state for the period selected by FBLANK

(Table 6).

For a step-voltage change at REFIN1, the rate of

change of the output voltage is limited by the inductor

current ramp, the total output capacitance, the current

limit, and the load during the transition. The inductor

current ramp is limited by the voltage across the induc-

tor and the inductance. The total output capacitance

determines how much current is needed to change the

output voltage. Additional load current slows down the

output-voltage change during a positive REFIN1 volt-

age change, and speeds up the output-voltage change

during a negative REFIN1 voltage change. For fast pos-

itive output-voltage transitions, the current limit must be

greater than the load current plus the transition current:

ILIMIT >

ILOAD +

COUT

Adding a capacitor across REFIN1 and GND filters

noise and controls the rate of change of the REFIN1

voltage during dynamic transitions. With the additional

capacitance, the REFIN1 voltage slews between the

two set points with a time constant given by REQ x

CREFIN1, where REQ is the equivalent parallel resis-

tance seen by the slew capacitor. Looking at Figure 12,

the time constant for a positive REFIN1 voltage transi-

tion is:

ÏPOS

â¡R8 Ã (R9+R10)

â¢

â£

(R9+R10)

â¤

â¥

â¦

CREFIN1

and the time constant for a negative REFIN1 voltage

transition is:

ÏNEG

ââ

R8 Ã R9â

R8+R9 â â

CREFIN1

Linear Regulator (LDO)

The maximum input voltage for the linear regulator is

28V, while the minimum input voltage is determined by

the 800mV (max) dropout voltage (VLDOIN(MIN) =

VLDOOUT + VDROPOUT) at 50mA load. Bypass the lin-

ear regulatorâs output (LDOOUT) with a 4.7ÂµF or

greater capacitor, providing at least 1ÂµF per 5mA of

internal and external load on the linear regulator. The

LDO can source up to 100mA for powering the con-

troller or supplying a small external load.

For the MAX1540A, the linear regulator provides the 5V

bias supply that powers the gate drivers and analog

controller (Figure 1), providing stand-alone capability.

The linear regulatorâs input is internally connected to

the battery voltage input (LDOIN = V+), and the gate-

driver input supply is internally connected to the linear

regulatorâs output (VDD = LDOOUT). Figure 13 is the

internal linear-regulator functional diagram.

For the MAX1541, the linear regulator supports Dual

Mode operation to allow the selection of a 5V output

voltage without requiring external components (Figure

1). Connect FBLDO to GND for a fixed 5.0V output. The

linear regulatorâs output voltage can be adjusted from

1.25V to 5.5V using a resistive voltage-divider (Figure

12). The MAX1541 regulates FBLDO to a 1.25V feed-

back voltage. The adjusted output voltage is:

VLDOOUT

VFBLDO

âââ1+

R11â

R12 â â

where VFBLDO = 1.25V. If unused, disable the MAX1541

linear regulator by connecting LDOON to GND.

______________________________________________________________________________________ 33

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