LT3493-3 LINER [Linear Technology], LT3493-3 Datasheet - Page 15

• Manufacturer Part Number: LT3493-3
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Frequency Compensation
The LT3493-3 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3493-3 does not require the ESR of the output
capacitor for stability allowing the use of ceramic capaci-
tors to achieve low output ripple and small circuit size.
Figure 9 shows an equivalent circuit for the LT3493-3
control loop. The error amp is a transconductance ampli-
fier with finite output impedance. The power section,
consisting of the modulator, power switch and inductor, is
modeled as a transconductance amplifier generating an
output current proportional to the voltage at the V
Note that the output capacitor integrates this current, and
that the capacitor on the V
amplifier output current, resulting in two poles in the loop.
R
tor, the loop crossover occurs above the R
simple model works well as long as the value of the
C
provides a zero. With the recommended output capaci-
24V SUPPLY
IMPEDANCE
ENERGIZED
U
LOW
+
+
+
Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3493-3 is Connected to a Live Supply
C
U
node (C
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1µF
10µF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
C
W
+
) integrates the error
I
IN
1Ω
C
C
V
IN
C
zero. This
U
2.2µF
2.2µF
2.2µF
LT3493-3
LT3493-3
LT3493-3
C
node.
(8a)
(8b)
(8c)
20V/DIV
20V/DIV
20V/DIV
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. With a larger
ceramic capacitor (very low ESR), crossover may be lower
and a phase lead capacitor (C
divider may improve the phase margin and transient
5A/DIV
5A/DIV
5A/DIV
V
V
V
100pF
I
I
I
LT3493-3
IN
IN
IN
IN
IN
IN
C
60k
GND
R
C
C
V
C
0.7V
Figure 9. Model for Loop Response
1M
AMPLIFIER
–
+
ERROR
1.6A/V
g
m
DANGER!
RINGING V
ABSOLUTE MAXIMUM
RATING OF THE LT3493-3
300µA/V
=
CURRENT MODE
20µs/DIV
20µs/DIV
20µs/DIV
POWER STAGE
g
m
=
–
+
IN
780mV
MAY EXCEED
SW
FB
PL
R1
R2
3493-3 F09
) across the feedback
C
ESR
C1
PL
+
LT3493-3
3493-3 F10
OUT
C1
15
3493-3f