LT1374CFE データシートの表示（PDF） - Linear Technology

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LT1374CFE

4.5A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1374

APPLICATIONS INFORMATION

For high frequency switchers, the sum of ripple current

slew rates may also be relevant and can be calculated

from:

CATCH DIODE

The suggested catch diode (D1) is a 1N5821 Schottky, or

its Motorola equivalent, MBR330. It is rated at 3A average

forward current and 30V reverse voltage. Typical forward

voltage is 0.5V at 3A. The diode conducts current only

during switch off time. Peak reverse voltage is equal to

regulator input voltage. Average forward current in normal

operation can be calculated from:

Peak-to-peak output ripple voltage is the sum of a triwave

created by peak-to-peak ripple current times ESR, and a

square wave created by parasitic inductance (ESL) and

ripple current slew rate. Capacitive reactance is assumed

to be small compared to ESR or ESL.

Example: with VIN =10V, VOUT = 5V, L = 10µH, ESR = 0.1Ω,

ESL = 10nH:

This formula will not yield values higher than 3A with

maximum load current of 4.25A unless the ratio of input to

output voltage exceeds 3.4:1. The only reason to consider

a larger diode is the worst-case condition of a high input

voltage and overloaded (not shorted) output. Under short-

circuit conditions, foldback current limit will reduce diode

current to less than 2.6A, but if the output is overloaded

and does not fall to less than 1/3 of nominal output voltage,

foldback will not take effect. With the overloaded condi-

tion, output current will increase to a typical value of 5.7A,

determined by peak switch current limit of 6A. With

VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 5.7A:

This is safe for short periods of time, but it would be

prudent to check with the diode manufacturer if continu-

ous operation under these conditions must be tolerated.

20mV/DIV

VOUT AT

IOUT = 1A

VOUT AT

IOUT = 50mA

0.5A/DIV

0.5µs/DIV

INDUCTOR

CURRENT

AT IOUT = 1A

INDUCTOR

CURRENT

AT IOUT = 50mA

1374 F03

Figure 3. LT1374 Ripple Voltage Waveform

BOOST PIN CONSIDERATIONS

For most applications, the boost components are a 0.27µF

capacitor and a FMMD914 diode. The anode is connected

to the regulated output voltage and this generates a

voltage across the boost capacitor nearly identical to the

regulated output. In certain applications, the anode may

instead be connected to the unregulated input voltage.

This could be necessary if the regulated output voltage is

very low (< 3V) or if the input voltage is less than 5V.

Efficiency is not affected by the capacitor value, but the

capacitor should have an ESR of less than 1Ω to ensure

1374fd

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