Datasheet

6
LT1301
APPLICATIONS INFORMATION
WUU
U
I
IN
500mA/DIV
V
OUT
5V/DIV
V
SHDN
10V/DIV
200µs/DIV
V
IN
= 5V, V
OUT
= 12V
LT1300 F5
V
OUT
100mV/DIV
AC COUPLED
V
SW
10V/DIV
I
L
500mA/DIV
20µs/DIV
V
IN
= 5V, V
OUT
= 12V, L = 33µH
C
OUT
= 33µF, I
LOAD
= 90mA
LT1300 F4
Figure 4. Burst Mode Operation in Action
Figure 5. Start-Up Response
Output Voltage Selection
The LT1301 can be selected to 5V or 12V under logic
control or fixed at either by tying Select to ground or V
IN
respectively. It is permissible to tie Select to a voltage
higher than V
IN
as long as it does not exceed 10V.
Efficiency in 5V mode will be slightly less that in 12V mode
due to the fact that the diode drop is a greater percentage
of 5V than 12V. Since the bipolar switch in the LT1301 gets
its base drive from V
IN
, no reduction in switch efficiency
occurs when in 5V mode. When V
IN
exceeds the pro-
grammed output voltage the output will follow the input.
This is characteristic of the simple step-up or “boost”
converter topology. A circuit example that provides a
regulated output with an input voltage above or below the
output (known as a buck-boost or SEPIC) is shown in the
Typical Applications section.
Shutdown
The converter can be turned off by pulling SHDN (pin 3)
high. Quiescent current drops to 10µA in this condition.
Bias current of 8µA to 10µA flows into the pin (at 5V input).
It is recommended that SHDN not be left floating. Tie the
pin to ground if the feature is not used. SHDN can be driven
high even if V
IN
is floating.
I
LIM
Function
The LT1301’s current limit (I
LIM
) pin can be used for soft
start. Upon start-up, the LT1301 will draw maximum
current from the supply (about 1A) from the supply to
charge the output capacitor. Figure 5 shows V
OUT
and I
IN
waveforms as the device is turned on. The high current
flow can create IR drops along supply and ground lines
or cause the input supply to drop out momentarily. By
adding R1 and C3 as shown in Figure 6, the switch
current in the LT1301 is initially limited to 400mA until
the 15µA flowing out of the I
LIM
pin charges up C3. Input
current is held to under 500mA while the output voltage
ramps up to 12V as shown in Figure 7. R1 provides a
discharge path for the capacitor without appreciably de-
creasing peak switch current. When using the I
LIM
pin soft-
start mode a minimum load of a few hundred microam-
peres is recommended to prevent C3 from discharging, as
no current flows out of I
LIM
when the LT1301 is not
I
IN
500mA/DIV
200µs/DIV
V
OUT
5VDIV
V
SHDN
10V/DIV
LT1300 F5
V
IN
= 5V, V
OUT
= 12V
Figure 7. Startup Response Soft-Start Circuitry Added
Figure 6.
C2
33µF

C3
0.1µF

R1
1M

L1
33µH
D1
1N5817
47µF
SW
I
LIM
PGND
GND
SENSE
LT1301
SELECT
LT1301 F6
SHDN
SHUTDOWN
12V
V
IN
V
IN
3.3V OR 5V
+
+