MAX25262/MAX25263

Automotive, 65V 2A/3A Mini Buck Converter

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Analog Devices | 13

The internal spread spectrum is disabled if the devices are synchronized to an external clock. However, the devices do

not filter the input clock on the SYNC pin and pass any modulation (including spread spectrum) present on the driving

external clock.

SYNC Mode Selection

To enable the skip mode, drive the SYNC low. In skip mode, the high-side FET turns for a fixed adaptable on-time

(depending on VOUT, VSUP, and FSW). The high-side FET then turns off and the low-side FET turns on until the inductor

current falls to zero cross threshold. Once the low-side FET turns off by hitting the zero-crossing threshold, LX becomes

high impedance, and the output voltage keeps decreasing. When the output voltage or FB voltage is detected below the

set point, the new cycle starts by turning on the high-side FET again. In this way, the regulator switches only as needed

to service load to improve system efficiency.

To enable the forced PWM (FPWM) mode, drive the SYNC high. The FPWM mode prevents the regulator from entering

skip mode by disabling the zero-cross detection of the inductor current. The benefit of the FPWM mode is to keep the

switching frequency constant under all load conditions. However, the FPWM operation diverts a considerable amount of

the output current to PGND, reducing the efficiency under light-load conditions. The FPWM mode is useful to improve the

load-transient response and eliminate unknown frequency harmonics that can interfere with AM radio bands.

Over Current Protection

The MAX25262/MAX25263 have a cycle-by-cycle current limit and include a hiccup mode to prevent any damage from

overcurrent or short-circuit on the power channel. When the inductor current continuously hits the current limit at over

current, the output voltage starts decreasing. If the IC detects the output voltage drops below 70% of the programmed

regulation value, it turns off that channel. After waiting for about 10ms of hiccup time, the IC restarts that channel in case

overcurrent or short-circuit condition is removed.

Soft-Start

When the IC is enabled, the soft-start circuitry gradually ramps up the reference voltage during soft-start time to reduce

the input surge current during startup. Before the soft-start can be enabled, the BIAS voltage must exceed its UVLO

threshold (3.1V typ).

For the parts using an external divider or fixed internal output parts that have an output voltage higher or equal to 5V, the

2.1Mhz parts have a 2.75ms ramp time, and the 400kHz parts have a 3.6ms ramp time.

For fixed output parts that have an output voltage lower than 5V, the soft-start ramp time can be calculated as follows:

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Internal Gate Driver Supply

The gate drive for internal MOSFETs is driven by the internal BIAS voltage. In case the EXTVCC is not supplied externally

with a power supply in the given range, the IC uses an internal LDO at 5V for the MOSFET gate drive. In case of an

appropriate voltage on the EXTVCC, the internal LDO is turned OFF and the gate drive voltage is equal to the voltage at

the EXTVCC.

The MOSFET RdsON and gate capacitance charge depend heavily on the gate drive voltage. Thus, the efficiency and

thermal performance of the IC depend on the voltage provided on the EXTVCC. An external +5V supply on the EXTVCC

is recommended as that saves power dissipation in the internal LDO and enhances the MOSFETs for low RdsON.