FAN5350 Ver la hoja de datos (PDF) - ON Semiconductor

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FAN5350

3MHz, 600mA Step-Down DC-DC Converter in Chip-Scale and MLP Packaging

ON Semiconductor 

Operation Description

The FAN5350 is a step-dow n sw itching voltage regulator

that delivers a f ixed 1.82V f rom an input voltage supply of

2.7V to 5.5V. Using a proprietary architecture w ith

synchronous rectification, the FAN5350 is capable of

delivering 600mA at over 90% efficiency, w hile

maintaining a light load efficiency of over 80% at load

currents as low as 1mA. The regulator operates at a

nominal frequency of 3MHz at full load, w hich reduces the

value of the external components to 1µH for the output

inductor and 4.7µF for the output capacitor.

Control Scheme

The FAN5350 uses a proprietary non-linear, fixed-

frequency PWM modulator to deliver a fast load transient

response, w hile maintaining a constant sw itching

frequency over a w ide range of operating conditions. The

regulator performance is independent of the output

capacitor ESR, allow ing for the use of ceramic output

capacitors. Although this type of operation normally

results in a sw itching frequency that varies w ith input

voltage and load current, an internal frequency loop holds

the sw itching frequency constant over a large range of

input voltages and load currents.

For very light loads, the FAN5350 operates in

discontinuous current (DCM) single-pulse PFM mode,

w hich produces low output ripple compared w ith other

PFM architectures. Transition betw een PWM and PFM is

seamless, w ith a glitch of less than 14mV at VOUT during

the transition betw een DCM and CCM modes.

Combined w ith exceptional transient response

characteristics, the very low quiescent current of the

controller (<16µA) maintains high efficiency, even at very

light loads, w hile preserving fast transient response for

applications requiring very tight output regulation.

Enable and Soft Start

Maintaining the EN pin LOW keeps the FAN5350 in non-

sw itching mode in w hich all circuits are off and the part

draw s ~50nA of current. Increasing EN above its

threshold voltage activates the part and starts the soft-

start cycle. During soft start, the current limit is increased

in discrete steps so that the inductor current is increased

in a controlled manner. This minimizes any large surge

currents on the input and prevents any overshoot of the

output voltage.

Under-Voltage Lockout

When EN is high, the under-voltage lock-out keeps the

part f rom operating until the input supply voltage rises high

enough to properly operate. This ensures no misbehavior

of the regulator during start-up or shutdow n.

Current Limiting

A heavy load or short circuit on the output causes the

current in the inductor to increase until a maximum current

threshold is reached in the high-side sw itch. Upon

reaching this point, the high-side sw itch turns off,

preventing high currents f rom causing damage.

The peak current limit show n in Figure 16, ILIM(PK) is slightly

higher than the open-loop tested current limit, ILIM(OL), in the

Electrical Characteristics table. This is primarily due to the

effect of propagation delays of the IC current limit

comparator.

Thermal Shutdown

When the die temperature increases, due to a high load

condition and/or a high ambient temperature, the output

sw itching is disabled until the temperature on the die has

fallen sufficiently. The junction temperature at w hich the

thermal shutdow n activates is nominally 150°C w ith a

20°C hysteresis.

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