MBRS340 データシートの表示（PDF） - Linear Technology

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MBRS340

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current

Linear Technology 

LT3975

APPLICATIONS INFORMATION

The LT3975 limits its peak switch current in order to protect

itself and the system from overload and short-circuit faults.

The LT3975’s switch current limit (ILIM) is typically 5.4A at

low duty cycles and decreases linearly to 4.4A at DC = 0.8.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor:

( ) ∆IL

=

(1–

DC) •

L•

VOUT

fSW

+

VD

where fSW is the switching frequency of the LT3975, DC is

the duty cycle and L is the value of the inductor. Therefore,

the maximum output current that the LT3975 will deliver

depends on the switch current limit, the inductor value,

and the input and output voltages. The inductor value may

have to be increased if the inductor ripple current does

not allow sufficient maximum output current (IOUT(MAX))

given the switching frequency, and maximum input voltage

used in the desired application.

The optimum inductor for a given application may differ

from the one indicated by this simple design guide. A larger

value inductor provides a higher maximum load current and

reduces the output voltage ripple. If your load is lower than

the maximum load current, than you can relax the value of

the inductor and operate with higher ripple current. This

allows you to use a physically smaller inductor, or one with

a lower DCR resulting in higher efficiency. Be aware that if

the inductance differs from the simple rule above, then the

maximum load current will depend on the input voltage. In

addition, low inductance may result in discontinuous mode

operation, which further reduces maximum load current.

For details of maximum output current and discontinuous

operation, see Linear Technology’s Application Note 44.

Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),

a minimum inductance is required to avoid sub-harmonic

oscillations, see Application Note 19.

One approach to choosing the inductor is to start with

the simple rule given above, look at the available induc-

tors, and choose one to meet cost or space goals. Then

use the equations above to check that the LT3975 will be

able to deliver the required output current. Note again

that these equations assume that the inductor current is

continuous. Discontinuous operation occurs when IOUT

is less than ΔIL/2.

Current Limit Foldback and Thermal Protection

The LT3975 has a large peak current limit to ensure a 2.5A

max output current across duty cycle and current limit

distribution, as well as allowing a reasonable inductor

ripple current. During a short-circuit fault, having a large

current limit can lead to excessive power dissipation and

temperature rise in the LT3975, as well as the inductor and

catch diode. To limit this power dissipation, the LT3975

starts to fold back the current limit when the FB pin falls

below 0.8V. The LT3975 typically lowers the peak current

limit about 40% from 5.4A to 3.3A.

During start-up, when the output voltage and FB pin are low,

current limit foldback could hinder the LT3975’s ability to

start up into a large load. To avoid this potential problem,

the LT3975’s current limit foldback will be disabled until

the SS pin has charged above 2V. Therefore, the use of

a soft-start capacitor will keep the current limit foldback

feature out of the way while the LT3975 is starting up.

The LT3975 has thermal shutdown to further protect the

part during periods of high power dissipation, particularly

in high ambient temperature environments. The thermal

shutdown feature detects when the LT3975 is too hot

and shuts the part down, preventing switching. When the

thermal event passes and the LT3975 cools, the part will

restart and resume switching. A thermal shutdown event

actively discharges the soft-start capacitor.

Input Capacitor

Bypass the input of the LT3975 circuit with a ceramic capaci-

tor of X7R or X5R type. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7μF to 10μF ceramic capacitor is adequate to

bypass the LT3975 and will easily handle the ripple cur-

rent. Note that larger input capacitance is required when

a lower switching frequency is used (due to longer on

times). If the input power source has high impedance, or

there is significant inductance due to long wires or cables,

additional bulk capacitance may be necessary. This can

be provided with a low performance electrolytic capacitor.

3975f

15

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