LT3504 Просмотр технического описания (PDF) - Linear Technology

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LT3504

Quad 40V/1A Step-Down Switching Regulator with 100% Duty Cycle Operation

Linear Technology 

LT3504

Applications Information

Bear in mind that significant power dissipation occurs in

Q1 during an overvoltage event. The MOSFET junction

temperature must be kept below its absolute maximum

rating. For the overvoltage transient shown in Figure 10,

MOSFET Q1 conducts 0.5A (full load on all buck channels)

while withstanding the voltage difference between VSUPPLY

(180V peak) and VIN (33V). This results in a peak power of

74W. Since the overvoltage pulse in Figure 10 is roughly

triangular, average power dissipation during the transient

event (about 400ms) is approximately half the peak power.

As such, the average power is given by:

PAVG(W)

=

1

2

•

P

PEAK(W)

=

37W

In order to approximate the MOSFET junction temperature

rise from an overvoltage transient, one must determine

the MOSFET transient thermal response as well as the

MOSFET power dissipation. Fortunately, most MOSFET

transient thermal response curves are provided by the

manufacturer (as shown in Figure 11). For a 400ms pulse

duration, the FQB34N20L MOSFET thermal response

ZΘJC(t) is 0.65°C/W. The MOSFET junction temperature

rise is given by:

TRISE(°C) = ZθJC(t) • PAVG(W) = 24°C

Note that, by properly selecting MOSFET Q1, it is possible

to withstand even higher input voltage surges. Consult

manufacturer data sheets to ensure that the MOSFET

operates within its Maximum Safe Operating Area.

The application circuit start-up behavior is shown in

Figure 12. Resistor R2 pulls up on the gate of Q1, forcing

source-connected VIN to follow approximately 3V below

VSUPPLY. Once VIN reaches the LT3504’s 3.2V minimum

start-up voltage, the on-chip boost converter immedi-

ately regulates the VSKY rail 4.85V above VIN. Diode D3

and resistor R3 bootstrap Q1’s gate voltage to the VSKY,

fully enhancing Q1. This connects VIN directly to VSUPPLY

through Q1’s low resistance drain-source path. It should

be noted that, prior to VSKY being present, the minimum

input voltage is about 6.2V. However, with VSKY in regulation

and Q1 enhanced, the minimum run voltage drops to 3.2V,

permitting the LT3504 to maintain regulation through deep

input voltage dips Figure 13 shows all channels operating

down to the LT3504’s 3.2V minimum input voltage.

3504fa

20

For more information www.linear.com/LT3504

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