FAN5182_08 datasheet(11/19 Pages) FAIRCHILD

部品番号	FAN5182

部品情報	Adjustable Output, 1-, 2-, or 3-Phase Synchronous Buck Controller

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FAN5182 • Rev. 1.1.3

11

The latch-off function can be reset by removing and

reapplying VCC or by pulling the EN pin low briefly. To

disable the over-current latch-off function, the external

resistor connecting the DELAY pin and ground should

be removed and a high-value resistor (>1MΩ) should be

connected from the DELAY pin to VCC. This prevents

the delay capacitor from discharging, so the 1.8V

threshold can never be reached. This pull-up resistor

has some impact to the soft-start time because the

current through this resistor adds additional current to

the internal 20µA soft-start current.

During start-up, when the output voltage is below

200mV, a secondary current limit is activated. This is

necessary because the voltage swing of CSCOMP

cannot go below ground. This secondary current limit

clamps the COMP voltage to 2V.

An inherent, per-phase current limit protects individual

phases if one or more phases cease to function

because of a faulty component. This limit is based on

the maximum normal mode COMP voltage.

Power-Good Monitoring

The power-good comparator monitors the output

voltage via the FB pin. The PWRGD pin is an open-

drain output whose high level (when connected to a

pull-up resistor) indicates that the output voltage is

within the nominal limits specified in the Electrical

Characteristic table. PWRGD goes low if the output

voltage is outside the specified range or whenever the

EN pin is pulled low. Figure 10 shows the PWRGD

response when the input power supply is switched off.

Figure 10. Shutdown Waveforms

As part of the protection for the load and output

components of the supply, the PWM outputs are driven

low (turning on the low-side MOSFETs) when the output

voltage exceeds the crowbar trip point. This crowbar

action stops once the output voltage falls below the

reset threshold of approximately 650mV.

Turning on the low-side MOSFETs pulls down the

output as the reverse current builds up in the inductors.

If the output over-voltage is due to a short in the high-

side MOSFET, this crowbar action can trip the input

supply over-current protection or blow the input fuse,

protecting the load from damage.

Enable and UVLO

To begin switching, the input supply (VCC) to the

controller must be higher than the UVLO threshold, and

the EN pin must be higher than its logic threshold. If

UVLO is less than the threshold or the EN pin is logic

low, the FAN5182 is disabled. This holds the PWM

outputs at ground, shorts the delay capacitor to ground,

and holds the ILIMIT pin at ground.

In the application circuit, the ILIMIT pin should be

connected to the OD pins of the FAN5109 drivers.

Grounding the ILIMIT pin disables the drivers such that

both HDRV and LDRV hold low. This feature is

important in preventing fast discharge of the output

capacitors when the controller shuts off. If the driver

outputs are not disabled, a negative output voltage can

be generated due to high current discharged from the

output capacitors through the inductors.

FAN5182 in Single-Phase Applications

NOTE

: When the FAN5182 is configured for single-

phase applications, it is actually operating internally as

a two-phase controller. It therefore should be configured

as a two-phase controller with only one phase

populated externally.

To accomplish this, PWM3 needs be grounded (to

configure the FAN5182 as a two-phase controller) and

the clock frequency set to two times the required phase

switching frequency. PWM1 should be used to drive the

external phase electronics (driver and MOSFETs). The

SW2 and SW3 pins should be connected to ground to

minimize any potential spurious noise paths.

WARNING

: Do not connect PWM2 to ground.

As noted above, when using the FAN5182 in single-

phase applications, it is actually operating internally as

a two-phase controller and PWM2 may be switching.

FAN5182 as a Voltage-Mode Controller

The SW pins are used to measure the current flowing

through the bottom FET. This current information is

used to balance the phase currents in a multiphase

application and create an inner current-feedback loop in

the control loop, making the FAN5182 a current-mode

controller.

In single-phase applications where phase current

balance is not required, the current loop can be

defeated by disconnecting the SW pins from the output

FETs and shorting the SW pin to ground. This changes

the control loop from current-mode control to voltage-

mode control.

WARNING

: The compensation requirements for a

voltage-mode control design differ for current-mode

control design. The Application Information section of

this datasheet is for a current-mode control design. The

compensation section “Closed-Loop Compensation

Design” does not apply to voltage-mode designs.

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