MAX17083 Просмотр технического описания (PDF) - Maxim Integrated

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MAX17083

Low-Voltage, Internal Switch, Step-Down Regulator

Maxim Integrated 

Low-Voltage, Internal Switch,

Step-Down Regulator

curve, while higher values result in higher full-load effi-

ciency (assuming that the coil resistance remains fixed)

and less output voltage ripple. Penalties for using high-

er inductor values include larger physical size and

degraded load-transient response (especially at low

input-voltage levels).

SMPS POR, UVLO, and Soft-Start

Power-on reset (POR) occurs when VCC rises above

approximately 2.1V, resetting the undervoltage, over-

voltage, and thermal-shutdown fault latches. The VCC

input undervoltage lockout (UVLO) circuitry prevents

the switching regulators from operating if the 5V bias

supply (VCC) is below its 4V UVLO threshold.

Soft-Startup

The internal step-down controller starts switching and the

output voltages ramp up using soft-start. If the bias supply

voltage drops below the UVLO threshold, the controller

stops switching and disables the drivers (LX becomes

high impedance) until the bias supply voltage recovers.

Once the 5V bias supply and IN rise above their respec-

tive input UVLO thresholds, and EN is pulled high, the

internal step-down controller becomes enabled and

begins switching. The internal voltage soft-starts gradu-

ally increment the feedback voltage by approximately

25mV every 61 switching cycles. Therefore, OUT reach-

es its nominal regulation voltage 1833/fSW after the regu-

lator is enabled (see the Soft-Start Waveforms in the

Typical Operating Characteristics section).

SMPS Power-Good Output (POK)

POK is the open-drain output of the window comparator

that continuously monitors the output for undervoltage

and overvoltage conditions. POK is actively held low in

shutdown (EN = GND) and during soft-start. Once the

soft-start sequence terminates, POK becomes high

impedance as long as the output remains within ±10%

of the nominal regulation voltage set by FB. POK goes

low once the output drops 12% (typ) below or rises 12%

(typ) above its nominal regulation point, or the output is

shut down. For a logic-level POK output voltage, con-

nect an external pullup resistor between POK and VCC.

A 100kΩ pullup resistor works well in most applications.

SMPS Fault Protection

Output Overvoltage Protection (OVP)

If the output voltage rises above 112% (typ) of its nomi-

nal regulation voltage, the controller sets the fault latch,

pulls POK low, shuts down the regulator, and immedi-

ately pulls the output to ground through its low-side

MOSFET. Turning on the low-side MOSFET with 100%

duty cycle rapidly discharges the output capacitors and

clamps the output to ground. However, this commonly

undamped response causes negative output voltages

due to the energy stored in the output LC at the instant

of 0V fault. If the load cannot tolerate a negative voltage,

place a power Schottky diode across the output to act

as a reverse-polarity clamp. If the condition that caused

the overvoltage persists (such as a shorted high-side

MOSFET), the input source also fails (short-circuit fault).

Cycle VCC below 1V or toggle the enable input to clear

the fault latch and restart the regulator.

Output Undervoltage Protection (UVP)

Each MAX17083 includes an output undervoltage

(UVP) protection circuit that begins to monitor the out-

put once the startup blanking period has ended. If the

output voltage drops below 88% (typ) of its nominal

regulation voltage, the regulator pulls the POK output

low and begins the UVP fault timer. Once the timer

expires after 1600/fSW, the regulator shuts down, forc-

ing the high-side off and disabling the low-side MOS-

FET once the zero-crossing threshold has been

reached. Cycle VCC below 1V, or toggle the enable

input to clear the fault latch and restart the regulator.

Thermal-Fault Protection

The MAX17083 features a thermal-fault protection

circuit. When the junction temperature rises above

+160°C (typ), a thermal sensor activates the fault latch,

pulls down the POK output, and shuts down the regu-

lator. Toggle EN to clear the fault latch, and restart the

controllers after the junction temperature cools by

15°C (typ).

SMPS Design Procedure

(Step-Down Regulator)

Firmly establish the input voltage range and maximum

load current before choosing a switching frequency

and inductor operating point (ripple-current ratio). The

primary design trade-off lies in choosing a good switch-

ing frequency and inductor operating point, and the fol-

lowing four factors dictate the rest of the design:

• Input Voltage Range. The maximum value (VIN(MAX)),

and minimum value (VIN(MIN)) must accommodate

the worst-case conditions accounting for the input

voltage soars and drops. If there is a choice at all,

lower input voltages result in better efficiency.

• Maximum Load Current. There are two values to

consider. The peak load current (ILOAD(MAX)) deter-

mines the instantaneous component stresses and fil-

tering requirements and thus drives output-capacitor

selection, inductor-saturation rating, and the design of

the current-limit circuit. The continuous load current

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