LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION
Table 3
The boost diode can be connected to the input, although,
caremustbetakentopreventthe2xV boostvoltagefrom
IN
VALUE
(μH)
IRMS
(Amps)
DCR
(Ω)
HEIGHT
(mm)
PART NUMBER
Coiltcraft
exceeding the BOOST pin absolute maximum rating. The
additional voltage across the switch driver also increases
powerloss,reducingefficiency.Ifavailable,anindependent
supply can be used with a local bypass capacitor.
DO1608C-222
Sumida
2.2
2.4
0.07
2.9
CDRH3D16-1R5
CDRH4D18-1R0
CDC5D23-2R2
CR43-1R4
1.5
1.0
2.2
1.4
2.6
1.6
1.7
2.2
2.5
2.6
0.043
0.035
0.03
1.8
2.0
2.5
3.5
3.0
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.Thisvalueisthenguardbandedby2xforsecondary
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.
0.056
0.013
CDRH5D28-2R6
Toko
(D62F)847FY-2R4M
(D73LF)817FY-2R2M
2.4
2.2
2.5
2.7
0.037
0.03
2.7
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:
SHUTDOWN AND UNDERVOLTAGE LOCKOUT
IOUT V − VOUT
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
loadtothesourceandcancausethesourcetocurrentlimit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
(
IN
)
ID(AVG
=
)
V
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.
LT1765
BOOST PIN
V
SW
7μA
IN
INPUT
Formostapplications, theboostcomponentsarea0.18μF
capacitor and a CMDSH-3 diode. The anode is typically
connected to the regulated output voltage to generate a
1.33V
R1
R2
3μA
V
CC
OUTPUT
voltage approximately V
above V to drive the output
SHDN
OUT
IN
+
C1
stage.Theoutputdriverrequiresatleast2.7Vofheadroom
throughouttheonperiodtokeeptheswitchfullysaturated.
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.
GND
1765 F04
Figure 4. Undervoltage Lockout
1765fd
10
Linear [ Linear ]
