LT1110_15 Datasheet, PDF (11/16 Pages) Linear Technology – Micropower DC-DC Converter Adjustable and Fixed 5V, 12V

LT1110
APPLICATI S I FOR ATIO
dissipation inside the LT1110. Output voltage is deter-
mined by
( ) ^V_OUT
=
_{1 +}
R2
R1
220mV .
(23)
R3 programs switch current limit. This is especially im-
portant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time
each cycle. Under certain conditions the current in L1 can
build up to excessive levels, exceeding the switch rating
and/or saturating the inductor. The 220Ω resistor pro-
grams the switch to turn off when the current reaches
approximately 800mA. When using the LT1110 in step-
down mode, output voltage should be limited to 6.2V or
less. Higher output voltages can be accommodated by
inserting a 1N5818 diode in series with the SW2 pin
(anode connected to SW2).
Higher Current Step-Down Operation
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
current limit sense. When the voltage drop across R1
equals a V_BE, the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
the I_LIM pin. This also lowers the maximum drop across R1
to V_BE – V_D, increasing efficiency. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the “Inductor Selection Step-Down
Q1
R1
MJE210 OR
V_IN
0.3Ω
ZETEX ZTX789A L1
25V
V_OUT
MAX
R2
220
V_IN
I_L
R3
330
D1
1N5821
C2
SW1
C1
LT1110
R4
FB
GND SW2
R5
( ) V_OUT = 220mV
1
+
R4
R5
LT1110 • TA16
Figure 6. Q1 Permits Higher-Current Switching.
LT1110 Functions as Controller.
Converter” section with the following conservative ex-
pression for V_SW:
V _SW = V_R1 + V _SAT ≈ 0.9V.
(24)
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage.
Inverting Configurations
The LT1110 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, the arrangement is very similar to
a step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and V_OUTshould be less than 6.2V. More negative out-
put voltages can be accommodated as in the prior section.
+V_IN
R3
I_LIM V_IN SW1
+
C2
FB
LT1110
SW2
GND
L1
R1
D1 +
1N5818
C1
R2
Figure 7. Positive-to-Negative Converter
–V_OUT
LT1110 • TA03
In Figure 8, the input is negative while the output is
positive. In this configuration, the magnitude of the input
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
L1
D1
+
C1
+V_OUT
R1
+
C2
I_LIM
V _IN
SW1
LT1110
2N3906
AO
GND
FB
SW2
( ) ^R2
V_OUT =
R1
R2
220mV + 0.6V
–V_IN
LT1110 • TA04
Figure 8. Negative-to-Positive Converter
11