lt1765es8-trpbf Linear Technology Corporation, lt1765es8-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt1765es8-trpbf
• Description: Monolithic 3a, 1.25mhz Step-down Switching Regulators
• Manufacturer: Linear Technology Corporation

LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIO S I FOR ATIO
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
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 calcu-
lated from:
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 in-
crease 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 condi-
tions 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
stage. The output driver requires at least 2.7V of head-
room 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
10
I
D AVG
( )
=
I
OUT
(
V
U
IN
V
VALUE
IN
(µH)
2.2
1.5
1.0
2.2
1.4
2.6
2.4
2.2
−
OUT
V
U
OUT
above V
)
(Amps)
I
RMS
2.4
1.6
1.7
2.2
2.5
2.6
2.5
2.7
W
IN
to drive the output
0.043
0.035
0.056
0.013
0.037
DCR
0.07
0.03
0.03
(Ω)
U
HEIGHT
(mm)
2.9
1.8
2.0
2.5
3.5
3.0
2.7
3.0
supply is used. The boost diode can be connected to the
input, although, care must be taken to prevent the 2x V
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 appli-
cations. 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
circuit 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.
C1
INPUT
R1
R2
IN
SHDN
LT1765
Figure 4. Undervoltage Lockout
3µA
1.33V
7µA
GND
V
V
SW
CC
+
1765 F04
OUTPUT
1765fb
IN