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
IVV
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. 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.
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 V
OUT
above V
IN
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.
The boost diode can be connected to the input, although,
care must be taken to prevent the 2x V
IN
boost voltage from
exceeding the BOOST pin absolute maximum rating. The
additional voltage across the switch driver also increases
power loss, reducing effi ciency. 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 fi lm 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 effi ciency. 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
V
SW
INPUT
R1
1765 F04
OUTPUT
SHDN
V
CC
IN
LT1765
3μA
R2
C1
+
7μA
