LTC1174-3.3 Linear Technology, LTC1174-3.3 Datasheet - Page 9

• Manufacturer Part Number: LTC1174-3.3
• Description: High Efficiency Step-Down and Inverting DC/DC Converter
• Manufacturer: Linear Technology

A
LTC1174 Adjustable Applications
The LTC1174 develops a 1.25V reference voltage between
the feedback (pin 1) terminal and ground (see Figure 4). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1174.
Inverting Applications
The LTC1174 can easily be set up for a negative output
voltage. If – 5V is desired, the LTC1174-5 is ideal for this
application as it requires the least components. Figure 5
shows the schematic for this application. Note that the
output voltage is now taken off the GND pin. Therefore,
the maximum input voltage is now determined by the
difference between the absolute maximum voltage rating
and the output voltage. A maximum of 12V is specified in
PPLICATI
V
OUT
Figure 4. LTC1174 Adjustable Configuration
1 25 1
.
V
LTC1174
IN
Figure 3. Low-Battery Comparator
O
U
R4
R3
R
R
V
S
FB
2
1
3
1
I FOR ATIO
U
–
+
LTC1174
1.25V
REFERENCE
100pF
W
1174 F03
R2
R1
1174 F04
V
OUT
U
Figure 5, giving the circuit a 1.5V of headroom for V
Note that the circuit can operate from a minimum of 4V,
making it ideal for a 4 NiCad cell application. For a higher
output current circuit, please refer to the Typical Applica-
tions section.
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1174. These items are also illustrated graphically in
the layout diagram in Figure 6. Check the following in your
layout:
1. Is the Schottky catch diode closely connected between
2. Is the “+” plate of C
3. Is the 0.1 F V
4. Is the SHUTDOWN (pin 8) actively pulled to V
5. Is the I
ground (pin 4) and switch (pin 5)?
This capacitor provides the AC current to the internal
P-channel MOSFET.
between V
carries the high frequency peak currents.
normal operation? The SHUTDOWN pin is high imped-
ance and must not be allowed to float.
I
to float.
PGM
**
*
3
2
7
AVX TPSD476K016
COILTRONICS CTX50-4
pin is high impedance and must not be allowed
Figure 5. Positive-to-Negative 5V Converter
PGM
LB
I
LB
PGM
INPUT VOLTAGE
IN
OUT
LTC1174HV-5
IN
4V TO 12V
(pin 7) pulled either to V
GND
(pin 6) and ground (pin 4)? This capacitor
V
SHUTDOWN
LTC1174-3.3/LTC1174-5
IN
IN
4
6
decoupling capacitor closely conected
V
OUT
SW
IN
closely connected to V
8
1
5
MBRS140T3
+
50 H**
0.1 F
+
+
IN
or ground? The
2 47 F*
16V
2 47 F*
16V
LTC1174
IN
IN
V
–5V
45mA
1174 F05
(pin 6)?
OUT
during
9
IN
.