LT3796 데이터 시트보기 (PDF) - Linear Technology

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LT3796

100V Constant-Current and Constant-Voltage Controller with Dual Current Sense

Linear Technology 

LT3796

APPLICATIONS INFORMATION

off-time (see Figure 8) and the switching frequency define

the minimum and maximum duty cycle of the switch,

respectively. The following equations express the mini-

mum/ maximum duty cycle:

Min Duty Cycle = minimum on-time • switching

frequency

Max Duty Cycle = 1 – minimum off-time • switching

frequency

350

300

250

MIN ON-TIME

200

MIN OFF-TIME

150

100

50

0

–50 –25

0 25 50 75 100 125 150

TEMPERATURE (°C)

3796 F08

Figure 8. Typical Minimum On- and Off-Time

vs Temperature

When calculating the operating limits, the typical values

for on/off-time in the data sheet should be increased by

at least 100ns to allow margin for PWM control latitude,

GATE rise/fall times and SW node rise/fall times.

Setting Input Current Limit

The LT3796 has a standalone current sense amplifier. It can

be used to limit the input current. As shown in Figure 9,

the input current signal is converted to voltage output at

CSOUT pin. When the CSOUT voltage exceeds FB2 regula-

tion voltage, the GATE is pulled low, and the converter stops

switching. The input current limit is calculated as follows:

IIN

=

1.25

•

RIN1

ROUT • RSNS

For buck applications, filter components, RIN2(OPT) and

COPT, are recommended to be placed close to LT3796 to

suppress the substantial transient signal or noise at across

18

CSN and CSP pins. For boost and buck-boost applications,

RIN2(OPT) and COPT are not required.

+VSNS–

RSNS

IIN

TO LOAD

VIN

RIN2(OPT)

COPT

RIN1

CSN

VS

–

+

LT3796

CSP VS

CSOUT FB2

3796 F03

CFILT ROUT

Figure 9. Setting Input Current Limit

Thermal Considerations

The LT3796 is rated to a maximum input voltage of 100V.

Careful attention must be paid to the internal power dis-

sipation of the IC at higher input voltages to ensure that

a junction temperature of 150°C is not exceeded. This

junction limit is especially important when operating at

high ambient temperatures. The majority of the power dis-

sipation in the IC comes from the supply current needed to

drive the gate capacitance of the external power N-channel

MOSFET. This gate drive current can be calculated as:

IGATE = fSW • QG

A low QG power MOSFET should always be used when

operating at high input voltages, and the switching fre-

quency should also be chosen carefully to ensure that the

IC does not exceed a safe junction temperature. The internal

junction temperature, TJ of the IC can be estimated by:

TJ = TA + [VIN • (IQ + fSW • QG) •θJA]

where TA is the ambient temperature, IQ is the VIN operating

current of the part (2.5mA typical) and θJA is the package

thermal impedance (30°C/W for the TSSOP package). For

example, an application with TA(MAX) = 85°C, VIN(MAX) =

60V, fSW = 400kHz, and having a N-channel MOSFET with

3796f

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