Datasheet

LT3688
17
3688f
Table 4. Capacitor Vendors
Part Number
V
R
(V)
I
AVE
(A)
V
F
at 1A
(mV)
On Semiconductor
MBR0520L 20 0.5
MBR0540 40 0.5 620
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B0530W 30 0.5
B120 20 1 500
B130 30 1 500
B140HB 40 1
DFLS140 40 1.1 510
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3688 due to their piezoelectric
nature. When in Burst Mode operation, the LT3688’s
switching frequency depends on the load current, and
at very light loads the LT3688 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3688 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
Frequency Compensation
The LT3688 uses current mode control to regulate the
output, which simplifi es loop compensation. In particular,
the LT3688 does not require the ESR of the output capaci-
tor for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. Figure 3
shows an equivalent circuit for the LT3688 control loop. The
error amp is a transconductance amplifi er with fi nite output
impedance. The power section, consisting of the modulator,
power switch and inductor, is modeled as a transconduc-
tance amplifi er generating an output current proportional to
the voltage at the V
C
node. Note that the output capacitor,
C1, integrates this current, and that the capacitor on the
V
C
node (C
C
) integrates the error amplifi er output current,
resulting in two poles in the loop. R
C
provides a zero.
With the recommended output capacitor, the loop cross-
over 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 ca-
pacitor (C
PL
) across the feedback divider may improve the
phase margin and transient response. At minimum, use a
10pF phase lead capacitor to reduce noise injection to the
FB pin. If the output capacitor is different than the recom-
mended capacitor, stability should be checked across all
operating 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. Figure 4
shows the transient response when the load current is
stepped from 300mA to 600mA and back to 300mA.
–
+
800mV
V
C
LT3688
GND
3688 F03
R1
OUT
ESR
TANTALUM OR
ELECTROLYTIC
CERAMIC
ERROR
AMPLIFIER
CURRENT MODE
POWER ST
AGE
FB
R2
3M
R
C
80k
C
C
100pF
C1
C1
g
m
=
1.6A/V
+
C
PL
0.7V
–
+
g
m
=
300µA/V
Figure 3. Model for the Loop Response
Figure 4. Transient Load Response of the LT3688
Front Page Application as the Load Current is
Stepped from 300mA to 600mA
APPLICATIONS INFORMATION
V
OUT
100mV/DIV
I
LOAD
200mA/DIV
50µs/DIV
3688 F04