lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 17

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Audible Noise
Ceramic capacitors are small, robust and have very
low ESR. However, ceramic capacitors can sometimes
cause problems when used with the LT3695 due to their
piezoelectric nature. When in Burst Mode operation, the
LT3695’s switching frequency depends on the load current,
and at very light loads the LT3695 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3695 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 LT3695 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3695 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
and a resistor (R
dition, there may be a lower value capacitor in parallel.
This capacitor (C
frequency, and is required only if a phase-lead capacitor
(C
C
PL
LT3695
pin, as shown in Figure 1. Generally a capacitor (C
CURRENT MODE
) is used or if the output capacitor has high ESR.
POWER STAGE
g
C
m
F
= 1.25S
V
R
C
C
C
C
3M
g
Figure 1. Model for Loop Response
m
= 430µS
F
C
) is used to fi lter noise at the switching
) in series to ground are used. In ad-
GND
–
+
0.8V
SW
FB
R2
R1
ELECTROLITIC
TANTALUM
POLYMER
C
PL
OR
OR
+
ESR
3695 F01
OUTPUT
C1
CERAMIC
C1
C
)
Loop compensation determines the stability and transient
performance. Optimizing the design of the compensation
network depends on the application and type of output
capacitor. A practical approach is to start with one of the
circuits in this data sheet that is similar to your applica-
tion and tune the compensation network to optimize the
performance. Stability should then 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 1
shows an equivalent circuit for the LT3695 control loop.
The error amplifi er 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 cur-
rent proportional to the voltage at the V
the output capacitor integrates this current, and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. In
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor R
C
of the inductor is not too high and the loop crossover
frequency is much lower than the switching frequency.
A phase lead capacitor (C
may improve the transient response. Figure 2 shows the
transient response when the load current is stepped from
300mA to 650mA and back to 300mA.
C
Figure 2. Transient Load Response of the LT3695. A 3.3V
Typical Application with V
Stepped from 300mA to 650mA
. This simple model works well as long as the value
100mV/DIV
0.5A/DIV
I
V
LOAD
OUT
C
pin (C
IN
PL
20μs/DIV
= 12V as the Load Current is
C
) across the feedback divider
) integrates the error ampli-
C
C
pin. Note that
3695 F02
LT3695
in series with
17
OUT
3695f