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MAX1980

Quick-PWM Slave Controller with Driver Disable for Multiphase DC-DC Converter

Maxim Integrated 

Quick-PWM Slave Controller with

Driver Disable for Multiphase DC-DC Converter

Table 4. Operating Mode Truth Table

DD

VCC

GND

ILIM

High

(> 0.25V)

High

(> 0.25V)

DL

Switching

Low

MODE

Normal

Operation

Driver

Disable

COMMENTS

Low-noise, fixed-frequency, PWM operation. The inductor current

reverses with light loads.

Light load, single-phase operation. The MAX1980 disables the drivers

by forcing DL and DH low, effectively disabling the slave controller.

Low-power, standby mode (ICC + IDD = 20µA typ). DL and DH

X

GND

(< 0.25V)

Low

Standby

forced low, and the PWM controller disabled. However, the bias and

fault-protection circuitry remain active so the MAX1980 can

continuously monitor the ILIM input.

X = Don’t Care

When the slave controller’s current-limit voltage (VILIM) is

set through a resistive-divider between the master con-

troller’s reference and GND (see the Current-Limit

Circuitry section), the MAX1980 automatically enters low-

power standby mode when the master controller shuts

down. As the master’s reference powers down, the resis-

tive-divider pulls ILIM below 250mV, automatically acti-

vating the MAX1980’s low-power standby mode.

Current-Limit Circuitry

When the master’s inductor current exceeds its valley

current limit, the master extends its off time by forcing

DL high until the inductor current falls below the current-

limit threshold. Without a transition on the master’s low-

side gate driver, the slave cannot initiate a new on-time

pulse so the slave’s inductor current ramps down as

well, maintaining the current balance. Therefore, the

slave’s valley current limit only needs to protect the

slave controller if the current balance circuitry or the

master current limit fails. The slave’s ILIM input voltage

should be selected to properly adjust the master’s cur-

rent-limit threshold.

Dual-Mode ILIM Input

The current-limit input (ILIM) features dual-mode opera-

tion, serving as both the standby mode control input

and the current-limit threshold adjustment. The slave

controller enters a low-power standby mode when the

ILIM voltage (VILIM) is pulled below 250mV. For ILIM

voltages from 400mV to 1.5V, the current-limit threshold

voltage is precisely 0.1 ✕ VILIM. The current-limit volt-

age may be accurately set with a resistive voltage-

divider between the master controller’s reference and

GND, allowing the MAX1980 to automatically enter the

low-power standby mode.

Slave Current Limit

The slave current-limit circuit employs a unique “valley”

current-sensing algorithm. If the current-sense signal is

above the current-limit threshold, the MAX1980 will not ini-

tiate a new cycle (Figure 3). The actual peak inductor cur-

rent is greater than the current-limit threshold by an

amount equal to the inductor ripple current. Therefore, the

exact current-limit characteristic and maximum load

capability are a function of the current-limit threshold,

inductor value, and input voltage. The reward for this

uncertainty is robust, overcurrent sensing. When com-

bined with master controllers that contain output under-

voltage protection circuits, this current-limit method is

effective in almost every circumstance.

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 MAX1980 uses CS+ and CS- to differentially measure

the current across an external sense resistor (RCS) con-

nected between the inductor and output capacitors. This

configuration provides precise current 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 cur-

rent-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

connection.

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 tem-

perature coefficient causes inaccurate current limiting.

As a result, high current-limit thresholds are needed to

16 ______________________________________________________________________________________

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