LT3742EUF-TRPBF 查看數據表（PDF） - Linear Technology

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LT3742EUF-TRPBF

Dual, 2-Phase Step-Down Switching Controller

Linear Technology 

LT3742

Applications Information

characteristic to notice about this method is that if the

output of controller 1 goes out of regulation enough

to trip the power good comparator, controller 2 will be

disabled.

In Figure 2b, a slightly larger capacitor on RUN/SS2 delays

the turn-on of controller 2 with respect to controller 1. The

start-up waveforms for this method look very similar to

the one shown in Figure 6a, but here controller 2 is not

disabled if controller 1 goes out of regulation.

In Figure 2c, both RUN/SS pins share a single capacitor

and start up at the same time. By sharing the same soft-

start signal, this method provides ratiometric tracking of

the two outputs.

Undervoltage Lockout (UVLO)

An external resistor divider can be used to accurately

set the minimum input voltage at which the LT3742 will

operate. Figure 3 shows the basic UVLO operation. Once

the UVLO pin drops below 1.25V, an undervoltage lock-

out event is signaled, turning on a 3µA current source to

provide hysteresis.

During a UVLO event, both controllers and the gate drive

boost regulator are disabled. For the LT3742, all RUN/SS

pins are discharged to get ready for a new soft-start cycle.

For each controller that is enabled, it’s RUN/SS pin will

be held to 500mV until the input voltage rises above the

upper UVLO trip voltage. The UVLO function is only active

when one or more of the controllers are enabled using the

RUN/SS pin. The UVLO pin can not be used to directly

start the part. Do not leave the UVLO pin unconnected;

tie it to VIN if not used. A separate, less accurate, internal

undervoltage lockout will disable the LT3742 when VIN is

less than 2.5V.

The UVLO resistor values are chosen to give the desired

minimum operating voltage (VIN(MIN)) and the desired

amount of hysteresis (VHYST). The LT3742 will turn on

when the input voltage is above (VIN(MIN) + VHYST), and

once on, will turn off when VIN drops below VIN(MIN). Select

the value for RUV1 first, then select the value for RUV2.

RUV1

=

VHYST

3µA

RUV 2

=

RUV1

•

1.25V

VIN(MIN) – 1.25V

Input Voltage Range

The minimum input voltage is determined by either the

LT3742’s minimum operating voltage of 4V, UVLO or by

the output voltages of a given application. The LT3742 can

operate at 100% duty cycle, so if the input voltage drops

close to or equal to one of the output voltages, the control-

ler will go into low dropout operation (100% duty cycle).

The duty cycle is the fraction of the time the N-channel

MOSFET is on every switch cycle, and is determined by

the input and output voltages:

⎛

DC = ⎜

VOUT + VD

⎞

⎟

⎝ VIN – VDS + VD ⎠

where VD is the forward drop of the catch diode (~0.4V)

and VDS is the typical MOSFET voltage drop (~0.1V).

VIN

VIN

2

1.25V

+

RUV1

UVLO 3

–

RUV2

3µA

UVLO

3742 F03

Figure 3. Undervoltage Lockout

3742fa

11

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