MAX5083ATE datasheet(9/18 Pages) MAXIM

9 / 18 page

1.5A, 40V, MAXPower Step-Down

DC-DC Converters

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Soft-Start and Reference (SS)

SS is the 1.23V reference bypass connection for the

MAX5082/MAX5083 and also controls the soft-start

period. At startup, after VIN is applied and the internal

and external UVLO thresholds are reached, the device

enters soft-start. During soft-start, 15µA is sourced into

the capacitor (CSS) connected from SS to SGND caus-

ing the reference voltage to ramp up slowly. When VSS

reaches 1.23V the output becomes fully active. Set the

soft-start time (tSS) using the following equation:

where tSS is in seconds and CSS is in Farads.

Internal Charge Pump (MAX5082)

The MAX5082 features an internal charge pump to

enhance the turn-on of the internal MOSFET, allowing

for operation with input voltages down to 4.5V. Connect

a flying capacitor (CF) between C+ and C-, a boost

diode from C+ to BST, as well as a bootstrap capacitor

(CBST) between BST and LX to provide the gate-drive

voltage for the high-side n-channel DMOS switch.

During the on-time, the flying capacitor is charged to

VDVREG. During the off-time, the positive terminal of the

flying capacitor (C+) is pumped to two times VDVREG

and charge is dumped onto CBST to provide twice the

regulator voltage across the high-side DMOS driver.

Use a ceramic capacitor of at least 0.1µF for CBST and

CF located as close to the device as possible.

For applications that do not require a 4.5V minimum

input, use the MAX5083. In this device, the charge

123

MAX5083

1.23V

ON/OFF

LDO

REG

THERMAL

SHDN

REF

0.3V

CLK

ILIM

1.23V

REGOK

VREF

ISS

>1.23V ON

<1.11V OFF

COMP

SYNC

E/A

SSA

CPWM

RAMP

LOGIC

OSC

BOOTSTRAP

CONTROL

PCLK

BST

SGND

DVREG

PGND

SCLK

HIGH-SIDE

CURRENT

SENSE

REF_PFM

PFM

REF_ILIM

ILIM

OVERL

CLK

ILIM

OVERLOAD

MANAGEMENT

Figure 2. MAX5083 Simplified Block Diagram

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