LT1173CS8 Linear Technology, LT1173CS8 Datasheet - Page 9

• Manufacturer Part Number: LT1173CS8
• Description: IC DC/DC CONV ADJUSTBL OUT 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1173CS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 50 V
• Current - Output: 400mA
• Frequency - Switching: 23kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

keep V
and R2 set the output voltage according to the formula
Step-Down (Buck Mode) Operation
A step-down DC-DC converter converts a higher voltage
to a lower voltage. The usual hookup for an LT1173 based
step-down converter is shown in Figure 6.
When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to V
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to
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 LT1173. Output voltage is deter-
mined by
A
V
PPLICATI
IN
i
V
V
PEAK
OUT
OUT
+
FB
at the internal reference voltage of 1.245V. R1
C2
V
1
1
I
IN
Figure 6. Step-Down Mode Hookup
LIM
R3
100
R
R
R
R
O
LT1173
V
GND
2
2
1
1
V
SW
IN
U
L
S
1 245
1 245
SW2
SW1
.
.
FB
V
I FOR ATIO
OUT
U
V
V
.
.
t
D1
1N5818
ON
L1
.
W
IN
+
– V
C1
IN
SW
– V
U
SW
LT1173 • TA11
– V
R2
R1
. This
V
OUT
OUT
14
15
16
,
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 100
grams the switch to turn off when the current reaches
approximately 800mA. When using the LT1173 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).
Inverting Configurations
The LT1173 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
tive output voltages can be accomodated as in the prior
section.
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.
+V
IN
I
LIM
R3
Figure 7. Positive-to-Negative Converter
LT1173
GND
V
OUT
IN
SW2
SW1
should be less than 6.2V. More nega-
FB
D1
1N5818
L1
+
C1
R1
R2
resistor pro-
LT1173 • F07
–V
+
LT1173
OUT
C2
9