MAX1887 Ver la hoja de datos (PDF) - Maxim Integrated

Número de pieza

componentes Descripción

Fabricante

MAX1887

Quick-PWM Slave Controllers for Multiphase, Step-Down Supplies

Maxim Integrated 

Quick-PWM Slave Controllers for

Multiphase, Step-Down Supplies

IPEAK

ILOAD

ILIMIT

( ) ILIMIT(VALLEY) = ILOAD(MAX)

2 - LIR

2η

0

TIME

Figure 3. “Valley” Current-Limit Threshold Point

There also is a negative current limit that prevents

excessive reverse inductor currents when VOUT is sink-

ing current. The negative current-limit threshold is set to

approximately 150% of the positive current-limit thresh-

old, and tracks the positive current limit when ILIM is

adjusted.

The MAX1887/MAX1897 uses CS+ and CS- to differen-

tially measure the current across an external sense

resistor (RCS) connected between the inductor and out-

put capacitors. This configuration provides precise cur-

rent balancing, current limiting, and voltage positioning

with a 1% current-sense resistor. Reducing the sense

voltage decreases power dissipation but increases the

relative measurement error.

Carefully observe the PC board layout guidelines to

ensure that noise and DC errors don’t corrupt the current-

sense signals measured at CS+ and CS-. The IC should

be mounted relatively close to the current-sense resistor

with short, direct traces making a Kelvin sense connec-

tion.

Master Current-Limit Adjustment (LIMIT)

The Quick-PWM controllers that may be used as the

master controller typically use the low-side MOSFET’s

on-resistance as its current-sense element. This depen-

dence on a loosely specified resistance with a large

temperature coefficient causes inaccurate current limit-

ing. As a result, high current-limit thresholds are need-

ed to guarantee full-load operation under worst-case

conditions. Furthermore, the inaccurate current limit

mandates the use of MOSFETs and inductors with

excessively high current and power dissipation ratings.

The slave includes a precision current-limit comparator

that supplements the master’s current-limit circuitry.

The MAX1887/MAX1897 uses CM+ and CM- to differ-

entially sense the master’s inductor current across a

current-sense resistor, providing a more accurate cur-

rent limit. When the master’s current-sense voltage

exceeds the current limit set by ILIM in the slave (see

the Dual-Mode ILIM Input section), the open-drain cur-

rent-limit comparator pulls LIMIT low (Figure 2). Once

the master triggers the current limit, a pulse-width mod-

ulated output signal appears at LIMIT. This signal is fil-

tered and used to adjust the master’s current-limit

threshold.

High-Side, Gate Driver Supply (BST)

The gate drive voltage for the high-side, N-channel

MOSFET is generated by the flying capacitor boost cir-

cuit (Figure 4). The capacitor between BST and LX is

alternately charged from the external 5V bias supply

(VDD) and placed in parallel with the high-side MOSFET’s

gate-source terminals.

On startup, the synchronous rectifier (low-side MOSFET)

forces LX to ground and charges the boost capacitor to

5V. On the second half of each cycle, the switch-mode

power supply turns on the high-side MOSFET by closing

an internal switch between BST and DH. This provides

the necessary gate-to-source voltage to turn on the high-

side switch, an action that boosts the 5V gate drive signal

above the system’s main supply voltage (V+).

CBYP

V+

DBST

BST

(RBST)*

CBST

DH

LX

INPUT

(VIN)

NH

L

MAX1887

MAX1897

( )* OPTIONAL–THE RESISTOR REDUCES

THE SWITCHING-NODE RISE TIME.

Figure 4. High-Side Gate Driver Boost Circuitry

______________________________________________________________________________________ 17

Share Link: