LT3579IUFD-1-PBF データシートの表示（PDF） - Linear Technology

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LT3579IUFD-1-PBF

6A Boost/Inverting DC/DC Converter with Fault Protection

Linear Technology 

LT3579/LT3579-1

OPERATION

OPERATION – OVERVIEW

The LT3579 uses a constant-frequency, current mode

control scheme to provide excellent line and load regulation.

The part’s undervoltage lockout (UVLO) function, together

with soft-start and frequency foldback, offers a controlled

means of starting up. Fault features are incorporated in

the LT3579 to aid in the detection of output shorts, over-

voltage, and overtemperature conditions. Refer to the Block

Diagram (Figure 1) and the State Diagram (Figure 2) for

the following description of the part’s operation.

VIN

VIN

RUVLO1

1.33V –

SHDN

+

RUVLO2

(OPTIONAL)

11.6μA

AT 1.33V

GND

ACTIVE/

LOCKOUT

3579 F03

Figure 3. Configurable UVLO

OPERATION – START-UP

Several functions are provided to enable a very clean

start-up for the LT3579.

Precise Turn-On Voltage

The SHDN pin compares to an internal voltage reference to

give a precise turn on voltage level. Taking the SHDN pin

above 1.33V (typical) enables the part. Taking the SHDN pin

below 0.3V shuts down the chip, resulting in extremely low

quiescent current. The SHDN pin has 30mV of hysteresis

to protect against glitches and slow ramping.

Undervoltage-Lockout (UVLO)

The SHDN pin can also be used to create a configurable

UVLO. The UVLO function sets the turn on/off of the LT3579

at a desired input voltage (VINUVLO). Figure 3 shows how

a resistor divider (or single resistor) from VIN to the SHDN

pin can be used to set VINUVLO. RUVLO2 is optional. It may

be left out, in which case set it to infinite in the equation

below. For increased accuracy, set RUVLO2 ≤ 10kΩ. Pick

RUVLO1 as follows:

RUVLO1

=

VINUVLO – 1.33V

⎛

⎜

1.33V

⎝⎜ RUVLO2

⎞

⎟

⎠⎟

+

11.6µA

The LT3579 also has internal UVLO circuitry that disables

the chip when VIN < 2.3V (typical).

Soft-Start of Switch Current

The soft-start circuitry provides for a gradual ramp-up of

the switch current (refer to Commanded Switch Current vs.

SS in Typical Performance Characteristics). When the part

is brought out of shutdown, the external SS capacitor is

first discharged which resets the states of the logic circuits

in the chip. Then an integrated 250k resistor pulls the SS

pin to ~1.8V at a ramp rate set by the external capacitor

connected to the pin. Once SS gets to 1.8V, the CLKOUT

pin is enabled, and an internal regulator pulls the pin up

quickly to ~2.1V. Typical values for the external soft-start

capacitor range from 100nF to 1μF.

Soft-Start of External PMOS (if used)

The soft-start circuitry also gradually ramps up the GATE

pin pull-down current which allows an external PMOS

to slowly turn on (M1 in Block Diagram). The GATE pin

current increases linearly with SS voltage, with a maximum

current of 933μA when the SS voltage gets above 500mV.

Note that if the GATE pin voltage is less than 2V for SS

voltages exceeding 500mV, then the GATE pin impedance

to ground is 2kΩ. The soft turn on of the external PMOS

helps limit inrush current at start-up, making hot plugs

of the LT3579 feasible and safe.

35791f

10

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