Datasheet

11
LT1302/LT1302-5
APPLICATIONS INFORMATION
WUU
U
1ms/DIV 1302 F08b
V
OUT
100mV/DIV
AC COUPLED
Figure 8b. 3.3k Resistor from I
T
Pin to Ground Forces
LT1302 into Current Mode Regardless of Load. Audio
Frequency Component Eliminated
525mA
25mA
I
LOAD
INDUCTOR
CURRENT
1A/DIV
The I
T
pin cannot be used as a soft-start. Large capacitors
connected to the pin will cause erratic operation. If oper-
ating the device in Burst Mode, let the pin float. Keep high
dV/dt signals away from the pin.
Figure 8c details efficiency with and without the addition of
R1. Burst Mode operation keeps efficiency high at light
load with I
T
floating. Efficiency falls off at light load with
R1 added because the LT1302 cannot transition into Burst
Mode.
Layout
The high speed, high current switching associated with
the LT1302 mandates careful attention to layout. Follow
the suggested component placement in Figure 9 for proper
operation. High current functions are separated by the
package from sensitive control functions. Feedback resis-
tors R1 and R2 should be close to the feedback pin (pin4).
Noise can easily be coupled into this pin if care is not taken.
A small capacitor (100pF to 200pF) from FB to ground
provides a high frequency bypass. If the LT1302 is oper-
ated off a three-cell or higher input, R3 (2Ω to 10Ω) in
series with V
IN
is recommended. This isolates the device
from noise spikes on the input supply. Do not put in R3 if
the device must operate from a 2V input, as input current
will cause the voltage at the LT1302’s V
IN
pin to go below
2V. The 0.1µF ceramic bypass capacitor C3 (use X7R, not
Z5U) should be mounted as close as possible to the
package. When R3 is used, C3 should be a 1µF tantalum
unit. Grounding should be segregated as illustrated. C3’s
ground trace should not carry switch current. Run a
Figure 8c. 3.3k Resistor for I
T
to Ground Increases
Efficiency at Moderate Load, Decreases at Light Load
OUTPUT CURRENT (mA)
1
EFFICIENCY (%)
90
80
70
60
50
40
30
10 100 1000
1302 F08c
I
T
FLOATING
3.3k I
T
TO GND
Figure 9. Suggested Component Placement for LT1302
ally reaches audio frequencies, but at a much lighter load
than without the I
T
feature. At some input voltage/load
current combinations, some residual bursting may occur
at frequencies out of the audio band.
1302 F09
4
3
2
18
7
6
5
LT1302
C3
R3
2Ω
D1
C2
C1
V
OUT
V
IN
L1
+
+
200pFR1
R2
R
C
C
C
SHUTDOWN
GND (BATTERY AND LOAD RETURN)