Datasheet
LT3689/LT3689-5
18
3689fe
For more information www.linear.com/LT3689
Table 4. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT I
AVE
(mV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530
550
Diodes Inc.
B120
B130
B140
B0540W
B140HB
20
30
40
40
40
1
1
1
0.5
1
500
500
500
510
530
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3689 due to their piezoelectric
nature. When in Burst Mode operation, the LT3689’s
switching frequency depends on the load current, and at
very light loads the LT3689 can excite the ceramic capaci-
tor at audio frequencies, generating audible noise. Since
the LT3689 operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet. If this
noise is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Frequency Compensation
The LT3689 uses current mode control to regulate the
output, which simplifies loop compensation. In particular,
the LT3689 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 LT3689 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, C1, integrates this current, and that the
capacitor on the V
C
node (C
C
) integrates the error ampli-
fier 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.
APPLICATIONS INFORMATION
Figure 3. Model for the Loop Response
–
+
–
+
800mV
V
C
LT3689
GND
3689 F03
R1
OUT
ESR
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
FB
R2
3M
R
C
37k
C
C
100pF
C1
C1
g
m
=
300µA/V
g
m
=
1.2A/V
+
C
PL
0.7V