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MAX16909

36V, 220kHz to 1MHz Step-Down Converter

with Low Operating Current

Detailed Description

The MAX16909 is a constant-frequency, current-mode,

automotive buck converter with an integrated high-side

switch. The device operates with input voltages from 3.5V

to 36V and tolerates input transients from 3.5V up to 42V.

During undervoltage events, such as cold-crank condi-

tions, the internal pass device maintains 98% duty cycle.

The switching frequency is resistor programmable from

220kHz to 1MHz to allow optimization for efficiency, noise,

and board space. A synchronization input FSYNC allows

the device to synchronize to an external clock frequency.

During light-load conditions, the device enters skip mode

for high efficiency. The 5V fixed output voltage eliminates

the need for external resistors and reduces the supply

current to 30FA. See the Internal Block Diagram for more

information.

Wide Input Voltage Range (3.5V to 36V)

The device includes two separate supply inputs, SUP

and SUPSW, specified for a wide 3.5V to 36V input

the device is operating with a 3.5V input supply, certain

conditions such as cold crank can cause the voltage at

SUPSW to drop below the programmed output voltage.

As such, the device operates in a high duty-cycle mode

to maintain output regulation.

Linear Regulator Output (BIAS)

The device includes a 5V linear regulator, BIAS, that

provides power to the internal circuitry. Connect a 1FF

ceramic capacitor from BIAS to GND.

External Clock Input (FSYNC)

The device synchronizes to an external clock signal

applied at FSYNC. The signal at FSYNC must have a

10% higher frequency than the internal clock frequency

for proper synchronization.

Soft-Start

The device includes an 8.5ms fixed soft-start time for up

to 500FF capacitive load with a 3A resistive load.

Minimum On-Time

The device features a 80ns minimum on-time that

ensures proper operation at 1MHz switching frequency

and high differential voltage between the input and the

output. This feature is extremely beneficial in automo-

tive applications where the board space is limited and

the converter needs to maintain a well-regulated output

voltage using an input voltage that varies from 9V to

18V. Additionally, the device incorporates an innovative

design for fast-loop response that further ensures good

output-voltage regulation during transients.

System Enable (EN)

An enable-control input (EN) activates the device from its

low-power shutdown mode. EN is compatible with inputs

from automotive battery level down to 3.3V. The high-

voltage compatibility allows EN to be connected to SUP,

KEY/KL30, or the INH pin of a CAN transceiver.

controller activates and the output voltage ramps up

within 8.5ms.

A logic-low at EN shuts down the device. During shut-

down, the internal linear regulator and gate drivers turn

off. Shutdown is the lowest power state and reduces the

quiescent current to 5FA (typ). Drive EN high to bring the

device out of shutdown.

Overvoltage Protection

The device includes overvoltage protection circuitry that

protects the device when there is an overvoltage condi-

tion at the output. If the output voltage increases by more

than 110% of its set voltage, the device stops switching.

The device resumes regulation once the overvoltage

condition is removed.

Fast Load-Transient Response

Current-mode buck converters include an integrator

architecture and a load-line architecture. The integra-

tor architecture has large loop gain but slow transient

response. The load-line architecture has fast transient

response but low loop gain. The device features an inte-

grator architecture with innovative designs to improve

transient response. Thus, the device delivers high output-

voltage accuracy, plus the output can recover quickly

from a transient overshoot, which could damage other

on-board components during load transients.

Overload Protection

The overload protection circuitry is triggered when the

threshold. Under these conditions the device turns the

high-side FET off for 16ms and re-enters soft-start. If the

overload condition is still present, the device repeats the

cycle.