MAX15118 Datasheet, PDF(13/23 Page) Maxim Integrated Products – High-Efficiency, 18A, Current-Mode Synchronous Step-Down Regulator with Integrated Switches

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MAX15118

High-Efficiency, 18A, Current-Mode Synchronous

Step-Down Regulator with Integrated Switches

Error Amplifier

A high-gain transconductance error amplifier provides

accuracy for the voltage-feedback loop regulation.

Connect the necesÂsary compensation network between

COMP and GND (see the Compensation Design

Guidelines section). The error-amplifier transconduc-

tance is 1.2mS (typ). COMP clamp low is set to 0.97V

(typ), just below the slope ramp compensation valley,

helping COMP to rapidly return to the correct set point

during load and line transients.

Ground-Sense Amplifier

The MAX15118 features a ground-sense amplifier to pre-

vent output voltage droop under heavy load conditions.

Connect GSNS to the negative terminal of the load output

capacitor to properly Kelvin-sense the output ground.

Route the GSNS trace away from the switching nodes.

PWM Comparator

The PWM comparator compares the COMP voltage to

the current-derived ramp waveform (COMP voltage to LX

current transconductance value is 150A/V, typ). To avoid

instability due to subharmonic oscillations when the duty

cycle is around 50% or higher, a slope compensation

ramp is added to the current-derived ramp waveform.

The compensation ramp slope is designed to ensure

stable operation at any duty cycle up to 94%.

Overcurrent Protection and Hiccup Mode

When the converter output is shorted or the device is

overloaded, each high-side MOSFET current-limit event

turns off the high-side MOSFET and turns on the low-side

MOSFET. On each current-limit event (either high-side

or low-side) a 3-bit counter is incremented. The counter

is reset after three consecutive switching cycles that do

not reach the current limit. If the current-limit condition

persists, the counter fills up reaching eight events. The

control logic then keeps the low-side MOSFET turned on

until the inductor current is fully discharged to avoid high

currents circulating through the low-side body diode.

The control logic turns off both high-side and low-side

MOSFETs and waits for the hiccup period (1024 clock

cycles, typ) before attempting a new soft-start sequence.

The hiccup mode is also enabled during soft-start time.

Thermal Shutdown Protection

The MAX15118 contains an internal thermal sensor that

limits the total power dissipation to protect the device in

the event of an extended thermal fault condition. When

the die temperature exceeds +150NC (typ), the thermal

sensor shuts down the device, turning off the DC-DC

converter to allow the die to cool. After the die temperaÂ

ture falls by 20NC (typ), the device restarts.

Skip Mode Operation

The MAX15118 features selectable skip mode operation

when SKIP is conÂnected to EN. When in skip mode, the

LX output becomes high impedance when the induc-

tor current falls below 0.5A (typ). The inductor current

does not become negative. If during a clock cycle the

inductor current falls below the 0.5A threshold (during

off-time), the low-side turns off. At the next clock cycle,

if the output voltage is above set point, the PWM logic

keeps both high-side and low-side MOSFETs off. If

instead the output voltage is below the set point, the

PWM logic drives the high-side on until a reduced cur-

rent limit threshold (3.6A, typ) is reached. In this way

the system can skip cycles, reducing the frequency of

operation, and switches only as needed to service load

at the cost of an increase in output voltage ripple (see

the Skip Mode Frequency and Output Ripple section). In

skip mode, power dissipation is reduced and efficiency

is improved at light loads because power MOSFETs do

not switch at every clock cycle.

The MAX15118 automatically enters continuous mode

regardless of the state of SKIP when the load current

increases beyond the skip mode current limit.

Do not change the state of SKIP when EN is high.

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