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LT3694EUFD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
LT3694/LT3694-1
13
36941fb
quality (under damped) tank circuit. If the LT3694 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3694’s
maximum input voltage rating. See Linear Technology
Application Note 88 for more details.
Output Capacitor Selection
The output capacitor filters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3694’s
control loop. Because the LT3694 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
Output ripple can be estimated with the following
equations:
V
RIPPLE
=
∆I
L
8 • f • C
OUT
; Ceramic
V
RIPPLE
= ∆I
L
• ESR ; Electrolytic
where ΔI
L
is the peak-to-peak ripple current in the inductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
I
C(RMS)
=
∆
I
L
12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
C
OUT
> 10 • L •
I
LIM
V
OUT
2
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3694 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes. Because
loop stability and transient response depend on the value
of C
OUT
, this loss may be unacceptable. Use X7R and X5R
types instead.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Surge rated tantalum capacitors
or low ESR, organic, electrolytic capacitors intended for
power supply use are suitable. Choose a capacitor with a
sufficiently low ESR for the required output ripple. Because
the volume of the capacitor determines its ESR, both the
size and the value will be larger than a ceramic capacitor
that would give similar ripple performance. One benefit
is that the larger capacitance may give better transient
response for large changes in load current. Table 3 lists
several capacitor vendors.
Table 3. Low ESR Surface Mount Capacitors
SERIES TYPE MANUFACTURER
Ceramic Taiyo Yuden
www.t-yuden.com
TPM, TPS Ceramic, Tantalum AVX
www.avx.com
T494, T495,
T510, T520,
T525, T530,
A700
Ceramic, Tantalum,
Tantalum Organic Polymer,
Aluminum Organic Polymer
Kemet
www.kemet.com
POSCAP,
OS-CON
Tantalum Organic Polymer,
Aluminum Organic Polymer
Sanyo
www.sanyo.com
SP-CAP Ceramic,
Aluminum Organic Polymer
Panasonic
www.panasonic.com
Ceramic TDK
www.tdk.com
APPLICATIONS INFORMATION
LT3694/LT3694-1
14
36941fb
Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
I
D(AVG)
=I
OUT
•
V
IN
−
V
OUT
V
IN
Consider a diode with a larger current rating than I
D(AVG)
when the part must survive a shorted output. The DA pin
monitors the current in the diode and prevents the switch
from turning on at the beginning of a charge cycle if the
diode current is above the DA limit. Therefore, under
overload conditions, the average diode current will in-
crease to the average of the switch current limit and the
DA current limit.
Peak reverse voltage is equal to the regulator input voltage,
so use a diode with a reverse voltage rating greater than
the maximum input voltage. The internal OVLO can protect
the diode from excessive reverse voltage by shutting down
the regulator if the input voltage exceeds 38V. Table 4 lists
several Schottky diodes and their manufacturers.
Table 4. Schottky Diodes (40V, 3A)
PART NUMBER V
f
at 3A (V) OUTLINE MANUFACTURER
MBRS340
MBRD340
0.5
0.6
SMC
D-PAK
ON Semiconductor
www.onsemi.com
B340
SMB340
0.5
0.5
SMC
Powermite 3
Diodes, Inc.
www.diodes.com
CMSH3-40
CSHD3-40
0.5
0.65
SMC
D-PAK
Central Semiconductor
www.centralsemi.com
Frequency Compensation
The LT3694 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3694 does not require the ESR of the output capacitor for
stability, so the user is free to employ ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
C
pin, as shown in Figure 2. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In addi-
tion, there may be lower value capacitor in parallel. This
capacitor (C
F
) is not part of the loop compensation but
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor (C
PL
) is used or if
the output capacitor (C1) has high ESR.
APPLICATIONS INFORMATION
–
+
0.75V
SW
V
C
g
m
=
350µS
GND
3M
LT3694
36941 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 7.5S
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
Loop compensation determines the stability and transient
performance. The best values for the compensation net-
work depend on the application and in particular the type
of output capacitor. A practical approach is to start with
one of the circuits in this data sheet that is similar to your
application 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 2
shows an equivalent circuit for the LT3694 control loop.
The error amplifier is a transconductance amplifier with
finite output impedance.
LT3694/LT3694-1
15
36941fb
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
C1
pin. Note that the output capacitor
integrates this current, and that the capacitor on the V
C1
pin (C
C
) integrates the error amplifier 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
in series with C
C
. 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. A phase lead capaci-
tor (C
PL
) across the feedback divider may improve the
transient response.
Figure 3 shows the transient response when the load
current steps from 1A to 2.6A and back to 1A.
BST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram in Figure 1) are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.22μF capacitor will work well. Figure 4 shows three
ways to arrange the boost circuit. The BST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
APPLICATIONS INFORMATION
Figure 3. Transient Load Response of the LT3694
Front Page Application as the Load Current Is
Stepped from 1A to 2.6A. V
OUT
= 3.3V
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
GND
LT3694
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
D2
GND
LT3694
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
GND
LT3694
36941 FO4
(4a) For V
OUT
> 2.8V
(4b) For 2.5V < V
OUT
< 2.8V
(4c) For V
OUT
< 2.5V; V
IN(MAX)
= 7V
Figure 4. Three Circuits for Generating the Boost Voltage
36941 F03
I
L
1A/DIV
V
OUT
100mV/DIV
100µs/DIV
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BST pin
operation with 2.5V outputs, use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 4b). For lower output voltages, the
BIAS pin can be tied to the input (Figure 4c), or to another
supply greater than 2.8V. Tying BIAS to V
IN
reduces the
maximum input voltage to 7V. The circuit in Figure 4a is
more efficient because the BST pin current and BIAS pin
quiescent current comes from a lower voltage source.
One must also ensure that the maximum voltage ratings
of the BST and BIAS pins are not exceeded. The minimum
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LT3694EUFD#PBF

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Buy LT3694EUFD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36V, 2.6A Monolithic Buck Regulator With Dual LDO
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