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MAX1887

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

Maxim Integrated 

PIN

MAX1887 MAX1897

12

14

13

15

14

16

15

17

16

18

Quick-PWM Slave Controllers for

Multiphase, Step-Down Supplies

NAME

DH

LX

BST

V+

LIMIT

Pin Description (continued)

DESCRIPTION

High-Side Gate-Driver Output. DH swings from LX to BST.

Inductor Connection. Connect LX to the switched side of the inductor. LX serves as

the lower supply rail for the DH high-side gate driver.

Boost Flying-Capacitor Connection. Connect to an external capacitor and diode

according to the Standard Application Circuit (Figure 1). An optional resistor in

series with BST allows DH pullup current to be adjusted.

Battery Voltage Sense Connection. Connect V+ to the input power source. V+ is

used only for PWM one-shot timing (see the On-Time Control and Active Current

Balancing section).

Open-Drain Current-Limit Output. Connect to the master controller’s adjustable current-

limit input (ILIM) according to the Standard Application Circuit (Figure 1). When the

voltage across the master controller’s current-sense resistor (VCM+ - VCM-) exceeds the

current-limit threshold (VILIM/10), the MAX1887/MAX1897 pulls LIMIT low.

Table 1. Component Selection for Standard

Applications

COMPONENT

Output Voltage

Input Voltage Range

Maximum Load Current

Inductor (each phase)

Frequency

High-Side MOSFET

(NH, each phase)

Low-Side MOSFET

(NL, each phase)

CIRCUIT OF FIGURE 1

0.6V to 1.75V

7V to 24V

40A

0.6µH

Sumida CDEP134H-0R6 or

Panasonic ETQP6F0R6BFA

300kHz (TON = float)

International Rectifier

(2) IRF7811W

International Rectifier

(2) IRF7822 or

Fairchild (3) FDS7764A or

Input Capacitor (CIN)

Output Capacitor (COUT)

Current-Sense Resistors

(RCS and RCM)

Voltage Positioning Gain

(AVPS)

(6) 10µF 25V

Taiyo Yuden

TMK432BJ106KM or

TDK C4532X5R1E106M

(8) 270µF 2V

Panasonic EEFUE0E271R

1.5mΩ

2

Detailed Description

The MAX1887/MAX1897 step-down slave controllers

are intended for low-voltage, high-current, multiphase

DC-to-DC applications. The MAX1887/MAX1897 slave

controllers can be combined with any of Maxim’s

Quick-PWM step-down controllers to form a multiphase

DC-to-DC converter. When compared to single-phase

operation, multiphase conversion lowers the peak

inductor current by distributing the load current

between parallel power paths. This simplifies compo-

nent selection, power distribution to the load, and ther-

mal layout. Existing Quick-PWM controllers, such as the

MAX1718, function as the master controller, providing

accurate output voltage regulation, fast transient

response, and multiple fault protection features.

Synchronized to the master’s low-side gate driver, the

MAX1887/MAX1897 include a constant on-time con-

troller, synchronous rectifier gate drive, active current

balancing, and precision current-limit circuitry.

On-Time Control and Active

Current Balancing

The MAX1887/MAX1897 slave controller uses a con-

stant on-time, voltage feed-forward architecture similar

to Maxim’s Quick-PWM controllers (Figure 2). The con-

trol algorithm is simple: the high-side switch on-time is

determined solely by a one-shot whose period is

inversely proportional to input voltage and directly pro-

portional to the compensation voltage (VCOMP).

Another one-shot sets a minimum off-time (130ns typi-

cal). The on-time one-shot is triggered when the follow-

ing conditions are satisfied: The slave detects a

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