LT1108 Datasheet, PDF (9/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
APPLICATI S I FOR ATIO
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
dissipation inside the LT1108. Output voltage is deter-
mined by
( ) ^V_OUT
=
_{1 +}
R2
R1
1.245V
(23)
R3 programs switch current limit. This is especially impor-
tant 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 100Ω resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1108 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).
V_IN
+
R3
100Ω
I_LIM
C2
V_IN SW1
FB
LT1108
SW2
GND
L1
D1 ⁺
1N5818
C1
V_OUT
R2
R1
Figure 2. Step-Down Mode Hookup
LT1108 • F02
HIGHER CURRENT STEP-DOWN OPERATION
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5V_BE, the switch turns off. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the Inductor Selection Step-Down
Converter section with the following conservative expres-
sion for V_SW:
V_SW = V_R1 + V_Q1SAT ≈ 1.0V
(24)
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V_IN is between 10V and 20V.
R1
Q1
V_IN
0.15Ω
ZETEX ZTX749
L1
30V
V_OUT
MAX
R2
100Ω
R6
100Ω
V_IN
I_L
R3
330Ω
C2
SW1
D1
1N5821
C1
LT1108
R4
FB
GND SW2
R5
( ) ^VOUT ^{= 1.245V}
1
+
R4
R5
LT1108 • F03
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
INVERTING CONFIGURATIONS
The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
(Figure 5). In Figure 4, 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 output
voltages can be accommodated as in the prior section.
In Figure 5, 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,
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