LT1304/LT1304-3.3/LT1304-5
U
OPERATIO
If the full power capability of the LT1304 is not required, bypass capacitor is required. If the input supply is close to
peakswitchcurrentcanbelimitedbyconnectingaresistor the IC, a 1µF ceramic capacitor can be used instead. The
RLIM from the ILIM pin to ground. With RLIM = 22k, peak LT1304switchescurrentin1Apulses, soalowimpedance
switchcurrentisreducedtoapproximately500mA.Smaller supplymustbeavailable.Ifthepowersource(forexample,
power components can then be used. The graph in Figure a 2 AA cell battery) is within 1 or 2 inches of the IC, the
6 shows switch current vs RLIM resistor value.
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
theIC,inductanceinthepowersourceleadsresultsinhigh
impedance at high frequency. A local high capacitance
bypassisthenrequiredtorestorelowimpedanceattheIC.
VOUT
2V/DIV
IIN
500mA/DIV
VSHDN
10V/DIV
SHUTDOWN
1ms/DIV
1304 F05
Figure 5. Start-Up Response with 1µF/1MΩ Components
in Figure 2 Added. Input Current Is More Controlled. VOUT
Reaches 5V in 6ms. Output Drives 20mA Load
1
2
3
4
8
7
6
5
LT1304
1000
900
800
700
600
500
400
V
IN
C
IN
+
V
OUT
C
OUT
+
GND (BATTERY AND LOAD RETURN)
1304 F07
10
100
(kΩ)
1000
R
LIM
Figure 7. Suggested Layout for Best Performance. Input
Capacitor Placement as Shown Is Highly Recommended.
Switch Trace (Pin 4) Copper Area Is Minimized
1304 F06
Figure 6. Peak Switch Current vs RLIM Value
Low-Battery Detector
Layout/Input Bypassing
TheLT1304containsanindependentlow-batterydetector
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 VIN reaches approximately
1.4V. Figure 8 illustrates the input/output characteristic of
the detector. Hysteresis is clearly evident in the figure.
The LT1304’s high speed switching mandates careful
attentiontoPCboardlayout.Suggestedcomponentplace-
mentisshowninFigure7. Theinputsupplymusthavelow
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
8
Linear [ Linear ]
