Single/Multiphase, Step-Down,

DC-DC Converter Delivers Up to 25A Per Phase

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13

Current-Limit Circuit

The current-limit threshold is set by a resistor between

ILIM and GND. Under soft-overload conditions, when

the peak inductor current exceeds the selected current

limit, the high-side MOSFET is turned off immediately

and the low-side MOSFET is turned on and remains on

to let the inductor current ramp down until the next

clock cycle. The converter does not stop switching and

the output voltage regulation is not guaranteed. Under

severe-overload or short-circuit conditions, the foldback

and hiccup current limit is simultaneously activated to

reduce power dissipation in the inductor, internal power

MOSFETs, and the upstream power source. Thus, the

circuit can withstand short-circuit conditions continu-

ously without causing overheating of any component. If

the device experiences a persistent overload condition,

the device will autoretry with a soft-start. The converter

will resume normal operation after the overload condi-

tion is removed.

The current-limit input is also used to communicate faults

between the devices in a multiphase configuration. With

any fault on the slave or master device (such as UVLO or

overtemperature), the ILIM input is pulled low, which

causes the other devices to turn off both MOSFETs.

Current Sharing

Accurate current sharing is required in a multiphase con-

verter to prevent some phases from overheating during

soft-start, steady-state, and load transient. For a convert-

er with current-mode control, the current is proportional

to the error-amplifier output in the voltage feedback loop.

The error-amplifier output (COMP) of the master is con-

nected to the current comparator input of all slave

devices. The current-sharing accuracy is determined by

the tolerances of the inductance and inductor DCR, the

input offset voltage, the gain of the current-sense ampli-

fiers, and the slope compensation circuits.

The peak current-mode control is an open-loop current-

sharing scheme, and therefore no compensation for

current sharing is needed and no stability issue exists.

Switching Frequency and

Ramp Generation (FREQ)

The MAX8686 has an adjustable internal oscillator that

can be set to any frequency from 300kHz to 1MHz. To set

the switching frequency, connect a capacitor from the

FREQ to GFREQ (see

Setting the Switching Frequency

section).

A triangle ramp from 0 to AVL/2 is generated across

FREQ capacitor. In a multiphase application, the

capacitor needs to be connected to the master device.

The FREQ inputs of the master and slave devices need

to be connected together. FREQ is internally pulled

down to GFREQ during shutdown.

Phase Selection Input (PHASE/REFO)

For single-phase or master device operation, the

PHASE/REFO can be used as a reference for the con-

verter output voltage (see the

Reference Output

(PHASE/REFO)/Reference Input (REFIN) section). For

multiphase operation, connect the PHASE/REFO of

each slave device to the center tap of the resistor-

divider from AVL of the master to GND. The resistor val-

ues are selected to set delay time between phases (see

the

Calculating the Phase Voltage section). The PWM

clock cycle of slave devices starts 60ns after the rising

edge of the voltage at FREQ crosses the voltage at

PHASE/REFO. The PWM clock cycle of the master

device starts at the beginning of the ramp.

Remote Sense Input (RS+, RS-)

For single-phase or master operation, connect RS+ to

the sense point at the load and RS- to the GND sense

point of the load. The connections should be at the out-

put regulation point to eliminate the voltage-sense error

caused by voltage drop between the device and load.

The RS+ and RS- traces should be laid out in parallel to

reduce noise coupling. A common-mode filter to each

sense trace should be added if further noise reduction

is needed.

For an output voltage higher than 3.3V, tie

PHASE/REFO to REFIN and use a resistor-divider from

the output regulation point to the remote sense inputs

(RS+, RS-), as shown in Figure 1b.

For multiphase operation, connect RS+ and RS- to AVL

(slave) to select the slave mode.

MAX8686

LOAD

RS+

REFIN

R1

R2

PHASE/REFO

RS-

Figure 1b. Output Voltage Above 3.3V