LT1110 Datasheet, PDF (10/16 Pages) Linear Technology – Micropower DC-DC Converter Adjustable and Fixed 5V, 12V

LT1110
APPLICATI S I FOR ATIO
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 is¹:
I_PEAK
=
V_{I N}
L
t _ON
L1
D1
V_IN
(20)
V_OUT
R3*
I_LIM
V _IN
SW1
LT1110 FB
R2
+
C1
GND
SW2
R1
* = OPTIONAL
Figure 4. Step-Up Mode Hookup.
LT1110 • TA14
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_OUT
=
_{1 +}
R2
R1
220mV .
(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.
V_IN
R3
220Ω
+
I_LIM
C2
V_IN SW1
FB
LT1110
SW2
GND
L1
D1 +
1N5818
C1
V_OUT
R2
R1
Figure 5. Step-Down Mode Hookup
LT1110 • TA15
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_PEAK
=
V_{IN −}
V_SW
L
−
V_OUT
t _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
Note 1: This simple expression neglects the effects of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
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