MAX669EUB/V Просмотр технического описания (PDF) - Maxim Integrated

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MAX669EUB/V

1.8V to 28V Input, PWM Step-Up Controllers in µMAX

Maxim Integrated 

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

Pin Description

PIN NAME

FUNCTION

1

LDO

5V On-Chip Regulator Output. This regulator powers all internal circuitry including the EXT gate driver.

Bypass LDO to GND with a 1µF or greater ceramic capacitor.

Oscillator Frequency Set Input. A resistor from FREQ to GND sets the oscillator from 100kHz (ROSC =

2

FREQ 500kΩ) to 500kHz (ROSC = 100kΩ). fOSC = 5 x 1010 / ROSC. ROSC is still required if an external clock is used

at SYNC/SHDN. (See SYNC/SHDN and FREQ Inputs section.)

3

GND Analog Ground

4

REF

1.25V Reference Output. REF can source 50µA. Bypass to GND with a 0.22µF ceramic capacitor.

5

FB

Feedback Input. The FB threshold is 1.25V.

6

CS+

Positive Current-Sense Input. Connect a current-sense resistor, RCS, between CS+ and PGND.

7

PGND Power Ground for EXT Gate Driver and Negative Current-Sense Input

8

EXT

External MOSFET Gate-Driver Output. EXT swings from LDO to PGND.

9

VCC

Input Supply to On-Chip LDO Regulator. VCC accepts inputs up to 28V. Bypass to GND with a 0.1µF ceramic

capacitor.

Shutdown control and Synchronization Input. There are three operating modes:

10

SYNC/

SHDN

• SYNC/SHDN low: DC-DC off.

• SYNC/SHDN high: DC-DC on with oscillator frequency set at FREQ by ROSC.

• SYNC/SHDN clocked: DC-DC on with operating frequency set by SYNC clock input. DC-DC conversion

cycles initiate on rising edge of input clock.

Detailed Description

The MAX668/MAX669 current-mode PWM controllers

operate in a wide range of DC-DC conversion applica-

tions, including boost, SEPIC, flyback, and isolated out-

put configurations. Optimum conversion efficiency is

maintained over a wide range of loads by employing

both PWM operation and Maxim’s proprietary Idle

Mode control to minimize operating current at light

loads. Other features include shutdown, adjustable

internal operating frequency or synchronization to an

external clock, soft start, adjustable current limit, and a

wide (1.8V to 28V) input range.

MAX668 vs. MAX669 Differences

Differences between the MAX668 and MAX669 relate

to their use in bootstrapped or non-bootstrapped cir-

cuits (Table 1). The MAX668 operates with inputs as

low as 3V and can be connected in either a boot-

strapped or non-bootstrapped (IC powered from input

supply or other source) configuration. When not boot-

strapped, the MAX668 has no restriction on output volt-

age. When bootstrapped, the output cannot exceed

28V.

The MAX669 is optimized for low input voltages (down

to 1.8V) and requires bootstrapped operation (IC pow-

ered from VOUT) with output voltages no greater than

28V. Bootstrapping is required because the MAX669

does not have undervoltage lockout, but instead drives

EXT with an open-loop, 50% duty-cycle start-up oscilla-

tor when LDO is below 2.5V. It switches to closed-loop

operation only when LDO exceeds 2.5V. If a non-boot-

strapped connection is used with the MAX669 and if

VCC (the input voltage) remains below 2.7V, the output

voltage will soar above the regulation point. Table 2

recommends the appropriate device for each biasing

option.

Table 1. MAX668/MAX669 Comparison

FEATURE

MAX668

MAX669

VCC Input

Range

3V to 28V

1.8V to 28V

Operation

Bootstrapped or nonboot-

strapped. VCC can be con-

nected to input, output, or

other voltage source such as

a logic supply.

Must be boot-

strapped (VCC

must be connect-

ed to boosted out-

put voltage, VOUT).

Under-

voltage

Lockout

IC stops switching for LDO

below 2.5V.

No

Soft-Start Yes

When LDO is

above 2.5V

8 _______________________________________________________________________________________

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