LT1765/LT1765-1.8/LT1765-2.5/

LT1765-3.3/LT1765-5

10

1765fd

APPLICATIONS INFORMATION

APPLICATIONS INFORMATION

Table 3

PART NUMBER

VALUE

(μH)

IRMS

(Amps)

DCR

(Ω)

HEIGHT

(mm)

Coiltcraft

DO1608C-222

2.2

2.4

0.07

2.9

Sumida

CDRH3D16-1R5

1.5

1.6

0.043

1.8

CDRH4D18-1R0

1.0

1.7

0.035

2.0

CDC5D23-2R2

2.2

2.2

0.03

2.5

CR43-1R4

1.4

2.5

0.056

3.5

CDRH5D28-2R6

2.6

2.6

0.013

3.0

Toko

(D62F)847FY-2R4M

2.4

2.5

0.037

2.7

(D73LF)817FY-2R2M

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:

I

IV

V

V

D AVG

OUT

IN

OUT

IN

−

()

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

worst-case condition of a high input voltage and shorted

output.Withashortedcondition,diodecurrentwillincreaseto

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.

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

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.

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

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.

Figure 4. Undervoltage Lockout

1.33V

GND

INPUT

R1

1765 F04

OUTPUT

SHDN

IN

LT1765

3μA

R2

C1

+

7μA