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output current is greater than a half of inductor ripple current. In both DCM and CCM, the output voltage is regulated by an

error amplifier. In case the on-time determined by a given operating condition in high output-voltage range (Mx_RNG[1:0]

= 0x2) is not long enough, the on-time automatically extends until the inductor current reaches 500mA to ensure enough

off-time to detect the ZX reliably.

Low-Power Skip (LP-Skip) Mode

Low-power skip mode {(Mx_LPM = 1 OR LPM_Mx = 1) AND Mx_FPWM = FPWM_Mx = 0} is similar to skip mode

because a negative inductor current is also not allowed in LP-skip mode. When the averaged output current is decreased

further down (> 8μs of LX three-state is detected two times consecutively) in skip mode, the buck converter enters LP-skip

mode when low-power mode is enabled. In LP-skip mode, the error amplifier and other internal blocks are deactivated

to reduce IQ consumption. Instead of the error amplifier, a low-power comparator monitors the output voltage in LP-skip

mode.

The buck enters DCM operation in skip mode when the duration of LX three-state is shorter than 4μs for eight times in a

row or LP-skip mode is disabled (Mx_LPM = LPM_Mx = 0). If no zero-crossing is detected (e.g., sudden load transient)

or FPWM mode is enabled (Mx_FPWM = 1 OR FPWM_Mx = 1), the buck enters CCM operation directly from LP-skip

mode.

LP-skip mode is not recommended for 4Φ configuration.

Forced-PWM (FPWM) Mode

Forced-PWM mode (Mx_FPWM = 1 OR FPWM_Mx = 1) ensures a continuous inductor current under all load conditions.

In FPWM mode, a negative inductor current through the low-side MOSFET is allowed but the maximum current is limited

to the INLIM (typ -3A). In case a valid external frequency is detected on the MFIOx input (when the CLKDET_Mx function

is selected), the corresponding buck enters FPWM mode regardless of its operating mode settings. See the Frequency

Tracking (FTRAK) section for more information.

Drop-Out Mode

The MAX77542 architecture allows the buck converter to operate even when the input voltage approaches the target

output voltage. When the headroom between the input and the output voltages reduces during operation, the buck

controller tries to maintain the output-voltage regulation by increasing the duty cycle. In case the buck is not able to

regulate the target output voltage with the maximum duty cycle (typ 98%), it automatically extends the on-time by skipping

the off-times (drop-out mode). In drop-out mode, the low-side MOSFET turns on occasionally to refresh the bootstrap

circuit for driving the high-side MOSFET. See the Bootstrap Refresh section for more information.

Switching Frequency

The MAX77542 has three nominal switching frequency options (0.5MHz, 1.0MHz, and 1.5MHz) to optimize the efficiency,

the transient response, the noise performance, and the solution size. The default switching frequency of the bucks are

set by the CFG2 input (see Table 3) and the switching frequencies of individual bucks are also selectable with the

Mx_FREQ[1:0] bits.

At any given time, the switching frequency (FSW) of the adaptive on-time buck converter is not fixed and is heavily

influenced by the instantaneous load current. More on-time pulses in a given time (higher FSW) are observed as the

output current increases, while fewer on-times in a given time (lower FSW) are observed when the output current

decreases. A valid external frequency at the MFIOx input (when the CLKDET_Mx function is selected) or enabling the

internal frequency tracking feature (Mx_FTRAK = 1) stabilizes the switching frequency of the corresponding buck in

steady-state operation. See the Frequency Tracking (FTRAK) section for more information.

In case the on-time calculated by the given operating condition is less than the minimum on-time (typ 90ns), the buck

controller regulates the output voltage by increasing the off-time. As a result, the actual switching frequency becomes

slower than its nominal frequency setting. For example, the calculated duty cycle for 16VIN and 0.8VOUT is about 5%,

which gives less than 90ns of on-time at 1MHz of nominal switching frequency. It means that the actual switching

frequency under this condition is slower than 1MHz. Therefore, a 0.5MHz of nominal switching frequency setting is

recommended.

MAX77542

16VIN/16A, Quad-Phase High-Efficiency Buck

Converter

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