Datasheet

9
LT1302/LT1302-5
APPLICATIONS INFORMATION
WUU
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behavior in the 4th graticule is the result of the LT1302’s
Burst Mode comparator turning off all switching as output
voltage rises above its threshold.
In Figure 7c, the 0.1µF capacitor has been replaced by a
0.01µF unit. Undershoot is less but the response is still
underdamped. Figure 7d shows the results of the 0.1µF
capacitor and a 10k resistor in series. Now some amount
of damping is observed, and behavior is more controlled.
Figure 7e details response with a 0.01µF/10k series net-
work. Undershoot is down to around 100mV, or 2%. A
slight underdamping is still noticeable.
Finally, a 0.01µF/24k series network results in the re-
sponse shown in Figure 7f. This has optimal damping,
undershoot less than 100mV and settles in less than 1ms.
The V
C
pin is sensitive to high frequency noise. Some
layouts may inject enough noise to modulate peak switch
current at 1/2 the switching frequency. A small capacitor
connected from V
C
to ground will eliminate this. Do not
exceed 1/10 of the compensation capacitor value.
Frequency Compensation
Obtaining proper RC values for the frequency compensa-
tion network is largely an empirical procedure, since
variations in input and output voltage, topology, capacitor
ESR and inductance make a simple formula elusive. As an
example, consider the case of a 2.5V to 5V boost converter
supplying 500mA. To determine optimum compensation,
the circuit is built and a transient load is applied to the
circuit. Figure 7 shows the setup.
In Figure 7a, the V
C
pin is simply left floating. Although
output voltage is maintained and transient response is
good, switch current rises instantaneously to the internal
current limit upon application of load. This is an undesir-
able situation as it places maximum stress on the switch
and the other power components. Additionally, efficiency
is well down from its optimal value. Next, a 0.1µF capacitor
is connected with no resistor. Figure 7b details response.
Although the circuit eventually stabilizes, the loop is quite
underdamped. Initial output “sag” exceeds 5%. Aberrant
Figure 7a. V
C
Pin Left Unconnected. Output Shows
Low Frequency Components Under Load
Figure 7b. 0.1µF from V
C
to Ground.
Better, but More Improvement Needed
2ms/DIV 1302 F07b
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
510mA
10mA
V
OUT
100mV/DIV
AC COUPLED
2ms/DIV
1302 F07a
I
LOAD
510mA
10mA
Figure 7. Boost Converter with Simulated Load
V
IN
I
T
SHDN
SENSE
SW
L1
10µH
D1
V
C
MTP3055EL
R
C
PULSE
GENERATOR
PGND
0.1µF
LT1302-5
GND
10Ω
2W
500Ω
NC
1302 F07
V
IN

2.5V
50Ω
+
C3
220µF
+
C2
220µF
C1, C2, C3 = AVX TPS SERIES
D1 = MOTOROLA MBRS130LT3
L1 = COILCRAFT DO3316-103K
+
C1
330µF