LT1765EFE-1.8-TR 데이터 시트보기 (PDF) - Linear Technology

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LT1765EFE-1.8-TR

Monolithic 3A, 1.25MHz Step-Down Switching Regulator

Linear Technology 

LT1765/LT1765-1.8/LT1765-2.5/

LT1765-3.3/LT1765-5

APPLICATIONS INFORMATION

Table 3

PART NUMBER

Coiltcraft

DO1608C-222

Sumida

CDRH3D16-1R5

CDRH4D18-1R0

CDC5D23-2R2

CR43-1R4

CDRH5D28-2R6

Toko

(D62F)847FY-2R4M

(D73LF)817FY-2R2M

VALUE IRMS DCR HEIGHT

(μH) (Amps) (Ω)

(mm)

2.2

2.4

0.07

2.9

1.5

1.6

0.043

1.8

1.0

1.7

0.035

2.0

2.2

2.2

0.03

2.5

1.4

2.5

0.056

3.5

2.6

2.6

0.013

3.0

2.4

2.5

0.037

2.7

2.2

2.7

0.03

3.0

CATCH DIODE

The diode D1 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:

( ) IOUT VIN − VOUT

ID( ) AVG =

VIN

The only reason to consider a larger than 3A diode is the

worst-case condition of a high input voltage and shorted

output. With a shorted condition, diode current will increase to

a typical value of 4A, determined by peak switch current limit

of the LT1765. A higher forward voltage will also limit switch

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

prudent to check with the diode manufacturer if continuous

operation under these conditions must be tolerated.

The boost diode can be connected to the input, although,

care must be taken to prevent the 2x VIN boost voltage from

exceeding the BOOST pin absolute maximum rating. The

additional voltage across the switch driver also increases

power loss, reducing efficiency. If available, an independent

supply can be used with a local bypass capacitor.

A 0.18μF boost capacitor is recommended for most ap-

plications. Almost any type of film or ceramic capacitor

is suitable, but the ESR should be <1Ω to ensure it can

be fully recharged during the off time of the switch. The

capacitor value is derived from worst-case conditions of

700ns on-time, 90mA boost current, and 0.7V discharge

ripple. This value is then guard banded by 2x for secondary

factors such as capacitor tolerance, ESR and temperature

effects. The boost capacitor value could be reduced under

less demanding conditions, but this will not improve cir-

cuit operation or efficiency. Under low input voltage and

low load conditions, a higher value capacitor will reduce

discharge ripple and improve start up operation.

SHUTDOWN AND UNDERVOLTAGE LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1765. Typically, UVLO is used in situations where

the input supply is current limited, or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so source current increases as

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. UVLO

prevents the regulator from operating at source voltages

where these problems might occur.

BOOST PIN

For most applications, the boost components are a 0.18μF

capacitor and a CMDSH-3 diode. The anode is typically

connected to the regulated output voltage to generate a

voltage approximately VOUT above VIN to drive the output

stage. The output driver requires at least 2.7V of headroom

throughout the on period to keep the switch fully saturated.

However, the output stage discharges the boost capacitor

during this on time. If the output voltage is less than 3.3V,

it is recommended that an alternate boost supply is used.

10

INPUT

LT1765

IN

R1

7μA

3μA 1.33V

SHDN

C1

R2

GND

VSW

VCC

+

OUTPUT

1765 F04

Figure 4. Undervoltage Lockout

1765fd

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