LT3502/LT3502A
18
3502fd
applications inForMation
Figure 10. Model for Loop Response
–
+
–
+
800mV
SW
V
C
LT3502
GND
3502 F10
R1
OUT
ESR
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
FB
R2
1M
R
C
150k
C
C
70pF
C1
C1
g
m
=
100µA/V
g
m
=
1A/V
+
C
PL
0.5V
the circuit. In Figure 9b an aluminum electrolytic capacitor
has been added. This capacitor’s high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of the
circuit, though it is likely to be the largest component in the
circuit. An alternative solution is shown in Figure 9c. A 1Ω
resistor is added in series with the input to eliminate the
voltage overshoot (it also reduces the peak input current).
A 0.1µF capacitor improves high frequency filtering. This
solution is smaller and less expensive than the electrolytic
capacitor. For high input voltages its impact on efficiency
is minor, reducing efficiency less than one half percent for
a 5V output at full load operating from 24V.
Frequency Compensation
The LT3502/LT3502A use current mode control to regulate
the output. This simplifies loop compensation. In particular,
the LT3502/LT3502A does not require the ESR of the output
capacitor for stability allowing the use of ceramic capacitors
to achieve low output ripple and small circuit size.
Figure 10 shows an equivalent circuit for the LT3502/
LT3502A control loop. The error amp is a transconductance
amplifier 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
C
node.
Note that the output capacitor integrates this current,
Figure 11
BST
DA
GND
D1
FB
V
IN
V
OUT
L1
C1
C2
C3
R1
R2
= VIA
3502 F11
BD
SHDN
and that the capacitor on the V
C
node (C
C
) integrates the
error amplifier output current, resulting in two poles in the
loop. R
C
provides a zero. With the recommended output
capacitor, the loop crossover occurs above the R
C
C
C
zero.
This simple model works well as long as the value of the
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
PL
) across the feedback
divider may improve the phase margin and transient
response. Large electrolytic capacitors may have an ESR
large enough to create an additional zero, and the phase
lead may not be necessary.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operat
-
ing conditions, including load current, input voltage and
temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load.
PCB Layout
For proper operation and minimum EMI, care must
be taken during printed cir
cuit board layout. Figure 11
shows the recommended component placement with
trace, ground plane and via locations. Note that large,
switched currents flow in the LT3502/LT3502A’s V
IN
and
SW pins, the catch diode (D1) and the input capacitor (C2).
