MAX1961 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1961

2.35V to 5.5V, 0.5% Accurate, 1MHz PWM Step-Down Controllers with Voltage Margining

Maxim Integrated 

2.35V to 5.5V, 0.5% Accurate, 1MHz PWM

Step-Down Controllers with Voltage Margining

Pin Description (continued)

PIN

MAX1960 MAX1961 MAX1962

NAME

15

15

15

C-

16

16

16

C+

17

17

17

VCC

18

18

18

BST

19

19

19

DH

20

20

20

LX

FUNCTION

Charge-Pump Flying Capacitor Negative Connection. Use a 0.47µF ceramic

capacitor at 1MHz, and 1µF between 450kHz and 950kHz.

Charge-Pump Flying Capacitor Positive Connection. Use a 0.47µF ceramic

capacitor at 1MHz and 1µF between 450kHz and 950kHz.

Input Supply to Charge Pump

Boost Capacitor Connection. Connect a 0.1µF ceramic capacitor from BST to LX.

High-Side MOSFET Gate-Driver Output. DH is low in shutdown.

Inductor Connection

Detailed Description

The MAX1960/MAX1961/MAX1962 are high-current,

high-efficiency voltage-mode step-down DC-DC con-

trollers that operate from 2.35V to 5.5V input and gener-

ate adjustable voltages down to 0.8V at up to 20A. An

on-chip charge pump generates a regulated 5V for dri-

ving a variety of external N-channel MOSFETs.

Constant frequency PWM operation and external syn-

chronization make these controllers suitable for telecom

and datacom applications. The operating frequency is

programmed externally to either 500kHz or 1MHz, or

from 450kHz to 1.2MHz with an external clock. A clock

output is provided to synchronize another converter for

180° out-of-phase operation.

A high closed-loop bandwidth provides excellent tran-

sient response for applications with dynamic loads.

Internal Charge Pump

An on-chip regulated charge pump develops 5V at

50mA (max) with input voltages as low as 2.35V. The

output of this charge pump provides power for the

internal circuitry, bias for the low-side driver (DL), and

the bias for the boost diode, which supplies the high-

side MOSFET gate driver (DH). The charge pump is

synchronized with the DL driver signal and operates at

1/2 the PWM frequency.

The external MOSFET gate charge is the dominant load

for the charge pump and is proportional to the PWM

switching frequency. The charge pump must supply

chip-operating current plus adequate gate current for

both MOSFETs at the selected operating frequency.

The required charge-pump output current is given by

the formula:

ITOTAL = IAVDD + fOSC (QG1 + QG2)

where IAVDD is the current supplied to the IC through

AVDD (typically 2mA), fOSC is the PWM switching

frequency, QG1 is the gate charge of the high-side

MOSFET, and QG2 is the gate charge of the low-side

MOSFET. The MOSFETs must be chosen such that

ITOTAL does not exceed 50mA. For example, with 1MHz

operation, QG1 + QG2 should be less than 48nC.

Voltage Margining and Shutdown

The voltage-margining feature on the MAX1960/

MAX1961 shifts the output voltage up or down by 4%.

This is useful for the automatic testing of systems at high

and low supply conditions to find potential hardware fail-

ures. CTL1 and CTL2 control voltage margining as out-

lined in Table 1.

A shutdown feature is included on all three parts, which

stops switching the output drivers and the charge

pump, reducing the supply current to less than 15µA.

For the MAX1962, drive EN high for normal operation,

or low for shutdown. For the MAX1960/MAX1961, drive

both CTL1 and CTL2 high for normal operation, or drive

CTL1 and CTL2 low for shutdown. For a simple

enable/shutdown function with no voltage margining,

connect CTL1 and CTL2 together and drive as one

input.

Table 1. Voltage Margining Truth Table

CTL1

High

High

Low

Low

CTL2

High

Low

High

Low

FUNCTION

Normal operation

+4% output-voltage shift

-4% output-voltage shift

Shutdown

10 ______________________________________________________________________________________

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