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MAX17761ATC

4.5V to 76V, 1A, High-Efficiency, Synchronous Step-Down DC-DC Converter

Maxim Integrated 

MAX17761

4.5V to 76V, 1A, High-Efficiency,

Synchronous Step-Down DC-DC Converter

Current Limit and Mode of Operation Selection

The following table lists the value of the resistors to program

PWM or PFM modes of operation and 1.6A or 1.14A peak

current limits.

The mode of operation cannot be changed on-the-fly after

power-up.

Table 1. RILIM Resistor vs.

Modes of Operation and Peak Current Limit

RILIM (kΩ)

MODE OF

OPERATION

PEAK CURRENT

LIMIT (A)

OPEN

PFM

1.6

422

PFM

1.14

243

PWM

1.6

121

PWM

1.14

PWM Mode Operation

In PWM mode, the inductor current is allowed to go negative.

PWM operation provides constant frequency operation at all

loads, and is useful in applications sensitive to switching fre-

quency. However, the PWM mode of operation gives lower effi-

ciency at light loads compared to the PFM mode of operation.

PFM Mode Operation

PFM mode of operation disables negative inductor current

and additionally skips pulses at light loads for high efficiency.

In PFM mode, the inductor current is forced to a fixed

peak every clock cycle until the output rises to 102% of

the nominal voltage. Once the output reaches 102% of

the nominal voltage, both the high side and low-side FETs

are turned off and the device enters hibernate operation

until the load discharges the output to 101% of the nominal

voltage. Most of the internal blocks are turned off in

hibernate operation to save quiescent current. After the

output falls below 101% of the nominal voltage, the device

comes out of hibernate operation, turns on all internal

blocks and again commences the process of delivering

pulses of energy to the output until it reaches 102% of the

nominal output voltage.

The advantage of the PFM mode is higher efficiency at

light loads because of lower quiescent current drawn

from supply. However, the output-voltage ripple is higher

compared to PWM mode of operation and switching

frequency is not constant at light loads.

Linear Regulator (VCC)

The MAX17761 has two internal low dropout regulators

(LDO), which power VCC. One LDO is powered from input

voltage and the other LDO is powered from the EXTVCC

pin. Only one of the two LDOs is in operation at a time,

depending on the voltage levels present at the EXTVCC pin.

If EXTVCC is greater than 4.74V (typ), VCC is powered from

the EXTVCC pin. If EXTVCC is lower than 4.44V (typ), VCC

is powered from input voltage. Powering VCC from EXTVCC

increases efficiency particularly at higher input voltages.

Typical VCC output voltage is 5V. Bypass VCC to SGND with

a 1µF cap. Both the LDOs can source up to 13mA.

When VCC falls below its undervoltage lockout (3.8V(typ)),

the internal step-down controller is turned off, and LX switch-

ing is disabled. The LX switching is enabled again when the

VCC voltage exceeds 4.2V (typ). The 400mV (typ) hyster-

esis prevents chattering on power-up/power-down.

When the EXTVCC is connected to the output and the

output is shorted such that inductive ringings cause the

output voltage to become temporarily negative, a R-C

network should be connected between the output and the

EXTVCC pin. A 4.7Ω between the output and the pin and

a 0.1µF from the pin to ground is recommended.

Switching Frequency Selection and External

Frequency synchronization

The RT/SYNC pin programs the switching frequency of

the converter. Connect a resistor from RT/SYNC to SGND

to set the switching frequency of the part at any one of

four discrete frequencies—200kHz, 300kHz, 400kHz, and

600kHz. Table 2 provides resistor values.

The internal oscillator of the device can be synchronized to

an external clock signal on the RT/SYNC pin. The external

synchronization clock frequency must be between 1.15 x

fSW and 1.4 x fSW, where fSW is the frequency programmed

by the resistor connected from the RT/SYNC pin.

Table 2. Switching Frequency vs.

RT Resistor

SWITCHING FREQUENCY

(kHz)

RT/SYNC RESISTOR

VALUE (kΩ)

200

210

300

140

400

105

600

69.8

Operating Input Voltage Range

The minimum and maximum operating input voltages for

a given output voltage should be calculated as follows:

VIN(MIN)

=

VOUT

+

(IOUT(MAX)

× (RDCR(MAX)

DMAX

+

R DS−ONL(MAX) ))

+ (IOUT(MAX) × (RDS−ONH(MAX) − RDS−ONL(MAX)))

VIN(MAX)

=

fSW(MAX)

VOUT

× t ON−MIN(MAX)

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