Datasheet

8
LT1304/LT1304-3.3/LT1304-5
OPERATIO
U
If the full power capability of the LT1304 is not required,
peak switch current can be limited by connecting a resistor
R
LIM
from the I
LIM
pin to ground. With R
LIM
= 22k, peak
switch current is reduced to approximately 500mA. Smaller
power components can then be used. The graph in Figure
6 shows switch current vs R
LIM
resistor value.
V
SHDN
10V/DIV
I
IN
500mA/DIV
bypass capacitor is required. If the input supply is close to
the IC, a 1µF ceramic capacitor can be used instead. The
LT1304 switches current in 1A pulses, so a low impedance
supply must be available. If the power source (for example,
a 2 AA cell battery) is within 1 or 2 inches of the IC, the
battery itself provides bulk capacitance and the 1µF ce-
ramic capacitor acts to smooth voltage spikes at switch
turn-on and turn-off. If the power source is far away from
the IC, inductance in the power source leads results in high
impedance at high frequency. A local high capacitance
bypass is then required to restore low impedance at the IC.
Figure 5. Start-Up Response with 1µF/1MΩ Components
in Figure 2 Added. Input Current Is More Controlled. V
OUT
Reaches 5V in 6ms. Output Drives 20mA Load
Layout/Input Bypassing
The LT1304’s high speed switching mandates careful
attention to PC board layout. Suggested component place-
ment is shown in Figure 7. The input supply must have low
impedance at AC and the input capacitor should be placed
as indicated in the figure. The value of this capacitor
depends on how close the input supply is to the IC. In
situations where the input supply is more than a few
inches away from the IC, a 47µF to 100µF solid tantalum
Figure 6. Peak Switch Current vs R
LIM
Value
R
LIM
(kΩ)
10
PEAK CURRENT (mA)
1000
900
800
700
600
500
400
100 1000
1304 F06

Low-Battery Detector
The LT1304 contains an independent low-battery detector
that remains active when the device is shut down. This
detector, actually a hysteretic comparator, has an open
collector output that can sink up to 500µA. The compara-
tor also operates below the switcher’s undervoltage lock-
out threshold, operating until V
IN
reaches approximately
1.4V. Figure 8 illustrates the input/output characteristic of
the detector. Hysteresis is clearly evident in the figure.
Figure 7. Suggested Layout for Best Performance. Input
Capacitor Placement as Shown Is Highly Recommended.
Switch Trace (Pin 4) Copper Area Is Minimized
1304 F07
8
7
6
54
3
2
1
LT1304
C
OUT
SHUTDOWN
V
OUT
V
IN
+
C
IN
+
GND (BATTERY AND LOAD RETURN)
1ms/DIV 1304 F05
V
OUT
2V/DIV