Datasheet

10
LT1302/LT1302-5
APPLICATIONS INFORMATION
WUU
U
2ms/DIV 1302 F07c
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
Figure 7c. 0.01µF from V
C
to Ground.
Underdamped Response Requires Series R
510mA
10mA
I
T
Pin
The I
T
pin is used to disable Burst Mode, forcing the
LT1302 to operate in current mode even at light load. To
disable Burst Mode, 3.3k resistor R1 is connected from I
T
to gound. More conservative frequency compensation
must be used when in this mode. A 0.1µF capacitor and
4.7k resistor from V
C
to ground has been found to be
adequate. Low frequency Burst Mode ripple can be
reduced or eliminated using this technique in many appli-
cations.
To illustrate, the transient load response of Figure 8’s
circuit is pictured without and with R1. Figure 8a shows
output voltage and inductor current without the resistor.
Note the 6kHz burst rate when the converter is delivering
25mA. By adding the 3.3k resistor, the low frequency
bursting is eliminated, as shown in Figure 8b. This feature
is useful in systems that contain audio circuitry. At very
light or zero load, switching frequency drops and eventu-
1ms/DIV 1302 F08a
V
OUT
100mV/DIV
AC COUPLED
Figure 8a. I
T
Pin Floating. Note 6kHz Burst Rate at
I
LOAD
= 25mA. 0.1µF/4.7k Compensation Network
Causes 220mV Undershoot
525mA
25mA
I
LOAD
INDUCTOR
CURRENT
1A/DIV
V
OUT
100mV/DIV
AC COUPLED
2ms/DIV 1302 F07f
510mA
10mA
I
LOAD
Figure 7f. 0.01µF, 24k Series RC
Results in Optimum Response
2ms/DIV 1302 F07e
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
510mA
10mA
Figure 7e. 0.01µF, 10k Series RC Shows Good
Transient Response. Slight Underdamping
Still Noticeable
2ms/DIV 1302 F07d
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
Figure 7d. 0.1µF with 10k Series RC.
Classic Overdamped Response
510mA
10mA
V
IN
SENSE
V
C
SW
10µH
V
OUT

5V
600mA
I
T
MBRS130LT3
PGND
0.1µF
LT1302-5
GND
4.7k
R1
3.3k
1302 F08
V
IN

2.5V
+
220µF
10V
+
220µF
10V
+
C1
330µF
0.1µF
Figure 8. Addition of R1 Eliminates Low Frequency
Output Ripple in This 2.5V to 5V Boost Converter