LT3640IUFD 查看數據表（PDF） - Linear Technology

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LT3640IUFD

Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer

Linear Technology 

LT3640

Applications Information

The maximum VIN should not exceed the absolute maxi-

mum rating. For fixed frequency operation, the maximum

VIN is:

VIN(MAX) =

 VOUT1 + VD 

 DCMIN 

− VD + VCE

Note that the high voltage buck will still regulate at an input

voltage that exceeds VIN(MAX) (up to 35V). However, the

switching frequency will be lowered to satisfy the equa-

tion (Figure 1).

Once the input voltage reaches 36.5V, an internal overvoltage

lockout (OVLO) circuit is triggered to disable switching ac-

tion (Figure 2). Without switching, the LT3640 can sustain

VIN voltage transients up to 55V for one second.

VIN2 Voltage Range

The low voltage synchronous channel operates from

the VIN2 pin. The VIN2 pin can be connected to either an

independent voltage supply or the high voltage channel

output for a two-stage power regulator.

In either configuration, if the high voltage channel is over-

loaded and pulled out of regulation, the low voltage channel

will be disabled. The SS2 pin will be discharged as well.

The minimum VIN2 voltage to regulate output voltage is:

VIN2(MIN)

≈

VOUT2

DCMAX

Where DCMAX is the maximum duty cycle (refer to

the Switching Frequency section). If VIN2 is below the

calculated minimum voltage, the output will fall out of

regulation.

The maximum VIN2 for fixed frequency operation is:

VIN2(MAX)

≈

VOUT2

DCMIN

Where DCMIN is the minimum duty cycle (refer to the Switch-

ing Frequency section). For voltage that exceeds VIN2(MAX)

(up to 5.5V), the low voltage channel exhi-bits pulse-skip-

ping behavior, and the output ripple will increase.

Inductor Selection

Inductor selection involves inductance, saturation current,

series resistance (DCR) and magnetic loss.

The inductance for the high voltage channel is:

L1 =

1.7

•

VOUT1 +

fS

VD

where VOUT1 is high voltage channel output voltage, VD

is the forward voltage drop of the catch diode, and fS is

the switching frequency. For example, 3.3µH is a reason-

able inductance for a 3.3V output with 2MHz switching

frequency.

Once the inductance is selected, the inductor current ripple

and peak current can be calculated:

∆IL1

=

(VOUT1 + VD)

IL1 • fS

•





1–

VOUT1 +

VIN

VD





IL(PEAK)

=

IOUT(MAX)

+

∆IL

2

SW1

10V/DIV

IL1

0.5A/DIV

200ns/DIV

VIN = 30V

RT SET = 2MHz

VOUT1 = 3.3V/0.2A

3640 F01

Figure 1. Lower Switching Frequency Occurs in High

Voltage Channel When Required On-Time Is Below 50ns

IL1

2A/DIV

VIN

20V/DIV

55VPK, 40V, 15V

10µs/DIV

3640 F02

Figure 2. VIN Overvoltage Lockout

3640f

13

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