LT1110
10
Diode Selection
Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1110
converters. General purpose rectifiers such as the 1N4001
are
unsuitable
for use in
any
switching regulator applica-
tion. Although they are rated at 1A, the switching time of
a 1N4001 is in the 10µs-50µs range. At best, efficiency will
be severely compromised when these diodes are used; at
worst, the circuit may not work at all. Most LT1110 circuits
will be well served by a 1N5818 Schottky diode, or its
surface mount equivalent, the MBRS130T3. The combina-
tion of 500mV forward drop at 1A current, fast turn ON and
turn OFF time, and 4µA to 10µA leakage current fit nicely
with LT1110 requirements. At peak switch currents of
100mA or less, a 1N4148 signal diode may be used. This
diode has leakage current in the 1nA-5nA range at 25°C
and lower cost than a 1N5818. (You can also use them to
get your circuit up and running, but beware of destroying
the diode at 1A switch currents.)
Step-Up (Boost Mode) Operation
A step-up DC-DC converter delivers an output voltage
higher than the input voltage. Step-up converters are
not
short circuit protected since there is a DC path from input
to output.
The usual step-up configuration for the LT1110 is shown
in Figure 4. The LT1110 first pulls SW1 low causing V
IN
–
V
CESAT
to appear across L1. A current then builds up in L1.
At the end of the switch ON time the current in L1 is
1
:
I
V
L
t
PEAK
IN
ON
= ()20
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Immediately after switch turn off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
OUT
+ V
D
, the
inductor current flows through D1 into C1, increasing
V
OUT
. This action is repeated as needed by the LT1110 to
keep V
FB
at the internal reference voltage of 220mV. R1
and R2 set the output voltage according to the formula
V
R
R
mV
OUT
=+
()
1
2
1
220 21.()
Step-Down (Buck Mode) Operation
A step-down DC-DC converter converts a higher voltage
to a lower voltage. The usual hookup for an LT1110 based
step-down converter is shown in Figure 5.
LT1110 • TA15
GND
SW2
SW1
LIM
I
IN
V
R3
220
FB
V
OUT
+
C2
+
C1
D1
1N5818
V
IN
R2
R1
L1
Ω
LT1110
Figure 5. Step-Down Mode Hookup
When the switch turns on, SW2 pulls up to V
IN
– V
SW
. This
puts a voltage across L1 equal to V
IN
– V
SW
– V
OUT
,
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to
I
V
VV
L
t
PEAK
IN
SW OUT
ON
=
−−
.()22
When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground.
D1
MUST BE A SCHOTTKY
DIODE
. The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
Figure 4. Step-Up Mode Hookup.
L1
LT1110 • TA14
GND SW2
SW1
LIM
I
IN
V
D1
R3*
LT1110
+
V
OUT
R2
R1
C1
* = OPTIONAL
V
IN
FB
Note 1: This simple expression neglects the effects of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
