lt3689-5 Linear Technology Corporation, lt3689-5 Datasheet - Page 18

• Manufacturer Part Number: lt3689-5
• Description: 700ma Step-down Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

LT3689/LT3689-5
APPLICATIONS INFORMATION
Table 4. Diode Vendors
PART NUMBER
On Semiconductor
MBRM120E
MBRM140
Diodes Inc.
B120
B130
B140
B0540W
B140HB
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.
18
(V)
V
20
40
20
30
40
40
40
R
LT3689
GND
I
(A)
0.5
AVE
1
1
1
1
1
1
V
R
37k
C
C
100pF
C
C
Figure 3. Model for the Loop Response
0.7V
3M
V
F
AMPLIFIER
(mV)
AT I
530
550
500
500
500
510
530
–
+
ERROR
1.2A/V
g
m
AVE
300μA/V
=
g
CURRENT MODE
m
POWER STAGE
=
–
+
800mV
Frequency Compensation
The LT3689 uses current mode control to regulate the
output, which simplifi es 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 amplifi er with fi nite
output impedance. The power section, consisting of the
modulator, power switch and inductor, is modeled as a
transconductance amplifi er generating an output current
proportional to the voltage at the V
output capacitor, C1, integrates this current, and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. R
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
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
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.
FB
R1
R2
C
PL
+
C
node (C
ESR
3689 F03
C1
OUT
PL
) across the feedback divider may
C
) integrates the error ampli-
C1
C
C
node. Note that the
C
C
zero. This simple
3689fa
C