MAX17761 查看數據表（PDF） - Maxim Integrated

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MAX17761

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

where:

VOUT = Steady-state output voltage

IOUT(MAX) = Maximum load current

RDCR(MAX) = Worse-case DC resistance of the inductor

fSW(MAX) = Maximum switching frequency

DMAX(MIN) = Minimum value of the maximum allowable

duty ratio (0.9)

tON-MIN(MAX) = Worst-case minimum switch on-time

(110ns)

RDS-ONL(MAX) and RDS-ONH(MAX) = Worst-case on-time

resistances of low-side and high-side internal MOSFETs,

respectively.

Overcurrent Protection

The device is provided with a robust overcurrent-protection

scheme that protects the device under overload and output

short-circuits conditions. The positive current limit is triggered

when the peak value of the inductor current hits a fixed

threshold (ILIM_P, 1.6A/1.14A, depending on the value

of the resistor connected to the MODE/ILIM pin). At this

point, the high-side switch is turned off and the low-side

switch is turned on. The low-side switch is kept on until the

inductor current discharges below 0.7 x ILIM_P.

While in PWM mode of operation, the negative current

limit is triggered when the valley value of the inductor

current hits a fixed threshold (ILIM_N, -0.65A/-0.455A,

depending on the value of the resistor connected to the

MODE/ILIM pin). At this point, the low-side switch is

turned off and the high-side switch is turned on.

RESET Output

The device includes RESET pin to monitor the output voltage.

The open-drain RESET output requires an external pullup

resistor. RESET goes high (high impedance) in 2.1ms after

the output voltage increases above 95% of the nominal volt-

age. RESET goes low when the output voltage drops to below

92% of the nominal voltage. RESET also goes low during

thermal shutdown.

Prebiased Output

When the device starts into a prebiased output, both the

high-side and low-side switches are turned off so that the

converter does not sink current from the output. High-

side and low-side switches do not start switching until

the PWM comparator commands the first PWM pulse, at

which point switching commences first with the high-side

switch. The output voltage is then smoothly ramped up to

the target value in alignment with the internal reference.

Thermal Shutdown Protection

Thermal shutdown protection limits total power dissipation

in the device. When the junction temperature of the device

exceeds +160°C, an on-chip thermal sensor shuts down

the device, allowing the device to cool. The thermal sensor

turns the device on again after the junction temperature

cools by 20°C. Soft-start resets during thermal shutdown.

Carefully evaluate the total power dissipation (see the

Power Dissipation section) to avoid unwanted triggering

of the thermal shutdown protection in normal operation.

Input Capacitor Selection

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching. The

input capacitor RMS current requirement (IRMS) for a

single output is defined by the following equation:

= IRMS IOUT ( MAX ) ×

VOUT × ( VIN − VOUT )

VIN

where, IOUT(MAX) = The maximum load current, IRMS

has a maximum value when the input voltage equals

twice the output voltage (VIN = 2 x VOUT), so IRMS(MAX)

= IOUT(MAX)/2.

Choose an input capacitor that exhibits less than +10°C

temperature rise at the RMS input current for optimal long-

term reliability. Use low-ESR ceramic capacitors with high-

ripple-current capability at the input. X7R capacitors are

recommended in industrial applications for their temperature

stability. Calculate the input capacitance using the following

equation:

CIN

=

IOUT(MAX) × D × (1−

η × fSW × ∆VIN

D)

where,

D = VOUT/VIN is the duty ratio of the controller,

fSW = The switching frequency,

ΔVIN = The allowable input voltage ripple,

η = The efficiency.

In applications where the source is located distant from

the device input, an electrolytic capacitor should be added

in parallel to the ceramic capacitor to provide necessary

damping for potential oscillations caused by the inductance

of the longer input power path and input ceramic capacitor.

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