Mode Selection (MODE)

The MAXM17543 features a MODE pin to configure the

device operating in PWM, PFM, or DCM control schemes.

The device operates in PFM mode at light loads if the

MODE pin is open. If the MODE pin connects to ground,

the device operates in constant-frequency PWM mode at

all loads. The device operates in constant-frequency DCM

State changes of the MODE operation are only at power-

up and ignore during normal operation.

PWM Mode Operation

In PWM mode, the step-down controller is switching

a constant-frequency at all loads with a minimum sink

current limit threshold (-1.8A typ) at light load. The

PWM mode of operation gives lower efficiency at light

loads compared to PFM and DCM modes of operation.

However, the PWM mode of operation is useful in applica-

tions sensitive to switching frequency.

PFM Mode Operation

In PFM mode, the controller forces the peak inductor

current in order to feed the light loads and maintain high

efficiency. If the load is lighter than the average PFM

value, the output voltage will exceed 102.3% of the feed-

back threshold and the controller enters into a hibernation

mode, turning off most of the internal blocks. The device

exits hibernation mode and starts switching again once the

output voltage is discharged to 101.1% of the feedback

threshold. The device then begins the process of delivering

pulses of energy to the output repeatedly until it reaches

102.3% of the feedback threshold. In this mode, the

behavior resembles PWM operation (with occasional pulse

skipping), where the inductor current does not need to

reach the light-load level.

PFM mode offers the advantage of increased efficiency

at light loads due to a lower quiescent current drawn from

the supply. However, the output-voltage ripple is also

increased as compared to the PWM or DCM modes of

operation, and the switching frequency is not constant at

light loads.

DCM Mode Operation

DCM mode features constant frequency operation down

to lighter loads than PFM mode, accomplished by not

skipping pulses. DCM efficiency performance lies between

the PWM and PFM modes.

External Frequency Synchronization (SYNC)

The device can be synchronized by an external clock

signal on the SYNC pin. The external synchronization

resistor. The minimum external clock high pulse width

and amplitude should be greater than 50ns and 2.1V,

respectively. The minimum external clock low pulse

width should be greater than 160ns, and the maximum

external clock low pulse amplitude should be less than 0.8V.

Table 1 provides recommended synchronous frequency

ranges for desired output voltages. Connect the SYNC

pin to SGND if it is not used.

RESET Output

The device includes a RESET comparator to monitor the

output for undervoltage and overvoltage conditions. The

open-drain RESET output requires an external pullup

voltage source. RESET goes high impedance after the

regulator output increases above 95% of the designed

nominal regulated voltage. RESET goes low when the

regulator output voltage drops below 92% of the nominal

regulated voltage. RESET also goes low during thermal

shutdown.

Thermal Fault Protection

The MAXM17543 features a thermal-fault protection

circuit. When the junction temperature rises above +165°C

(typ), a thermal sensor activates the fault latch, pulls down

the RESET output, and shuts down the regulator. The

thermal sensor restarts the controllers after the junction

temperature cools by 10°C (typ). The soft-start resets

during thermal shutdown.

Power Dissipation and Output-Current Derating

The MAXM17543 output current needs to be derated

if the device needs to be operated in a high ambient-

temperature environment. The amount of current-derating

depends upon the input voltage, output voltage, and

ambient temperature. The derating curves in TOC43

from the Typical Operating Characteristics section can be

used as guidelines. The curves are based on simulating

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Maxim Integrated │ 15

MAXM17543

4.5V to 42V, 2.5A High-Efficiency, DC-DC Step-Down

Power Module with Integrated Inductor