LT3782AIFE(RevC) データシートの表示（PDF） - Linear Technology

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LT3782AIFE
(Rev.:RevC)

2-Phase Step-Up DC/DC Controller

Linear Technology 

LT3782A

Applications Information

Operation

The LT3782A is a two phase constant frequency current

mode boost controller. Switching frequency can be pro-

grammed up to 500kHz. During normal switching cycles,

the two channels are controlled by internal flip-flops and

are 180 degrees out-of-phase.

Referring to the Block Diagram, the LT3782A’s basic func-

tions include a transconductance amplifier (gm) to regulate

the output voltage and to control the current mode PWM

current loop. It also includes the necessary logic and flip-

flop to control the PWM switching cycles, two high speed

gate drivers to drive high power N-channel MOSFETs, and

2-phase control signals to drive external gate drivers for

optional synchronous operation.

In normal operation, each switching cycle starts with a

switch turn-on. The inductor current of each channel is

sampled through the current sense resistor and amplified

then compared to the error amplifier output VC to turn

the switch off. The phase delay of the second channel is

controlled by the divide-by-two D flip-flop and is exactly

180 degrees out-of-phase of the first channel. With a re-

sistor divider connected to the FB pin, the output voltage

is programmed to the desired value. The 10V gate drivers

are sufficient to drive most high power N-channel MOSFET

in many industrial applications.

Additional important features include shutdown, cur-

rent limit, soft-start, synchronization and programmable

maximum duty cycle. Additional slope compensation can

be added also.

Output Voltage Programming

With a 2.44V feedback reference voltage VREF, the output

VOUT is programmed by a resistor divider as shown in

the Block Diagram.

VOUT

=

⎛

2.44⎜1+

⎝

RF1

RF 2

⎞

⎟

⎠

Soft-Start and Shutdown

During soft-start, the voltage on the SS pin (VSS) controls

the output voltage. The output voltage thus ramps up fol-

lowing VSS. The effective range of VSS is from 0V to 2.44V.

The typical time for the output to reach the programmed

level is:

t

=

C

• 2.44V

10µA

C is the capacitor connected from the SS pin to GND.

Undervoltage Lockout and Shutdown

Only when VRUN is higher than 2.45V VGBIAS will be active

and the switching enabled. The LT3782A goes into low

current shutdown when VRUN is below 0.3V. A resistor

divider can be used on RUN pin to set the desired VCC

undervoltage lockout voltage. 80mV of hysteresis is built

in on RUN pin thresholds.

Oscillation Frequency Setting and Synchronization

The switching frequency of LT3782A can be set up to

500kHz by a resistor RFREQ from pin RSET to ground.

For fSET = 250kHz, RFREQ = 80k

Once the switching frequency fSET is chosen, RFREQ can be

found from the Switching Frequency vs RFREQ graph found

under the Typical Performance Characteristics section.

Note that because of the 2-phase operation, the internal

oscillator is running at twice the switching frequency. To

synchronize the LT3782A to the system frequency fSYSTEM,

the synchronizing frequency fSYNC should be two times

fSYSTEM, and the LT3782A switching frequency fSET should

be set below 80% of fSYSTEM.

fSYNC = 2fSYSTEM and fSET < (fSYSTEM • 0.8)

For example, to synchronize the LT3782A to 200kHz sys-

tem frequency fSYSTEM, fSYNC needs to be set at 400kHz

and fSET needs to be set at 160kHz. From the Switching

Frequency vs RFREQ graph found under the Typical Per-

formance Characteristics section, RFREQ = 130k.

3782afc

8

For more information www.linear.com/LT3782A

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