lt3694-1 Linear Technology Corporation, lt3694-1 Datasheet - Page 14

• Manufacturer Part Number: lt3694-1
• Description: Lt3694/lt3694-1 - 36v, 2.6a Monolithic Buck Regulator With Dual Ldo
• Manufacturer: Linear Technology Corporation

LT3694/LT3694-1
APPLICATIONS INFORMATION
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:
Consider a diode with a larger current rating than I
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
MBRS340
MBRD340
B340
SMB340
CMSH3-40
CSHD3-40
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
14
I
D(AVG)
=I
OUT
V
f
at 3A (V)
•
0.65
0.5
0.6
0.5
0.5
0.5
V
IN
− V
V
IN
OUT
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OUTLINE
D-PAK
D-PAK
SMC
SMC
SMC
ON Semiconductor
www.onsemi.com
Diodes, Inc.
www.diodes.com
Central Semiconductor
www.centralsemi.com
MANUFACTURER
D(AVG)
V
and a resistor (R
tion, there may be lower value capacitor in parallel. This
capacitor (C
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor (C
the output capacitor (C1) has high ESR.
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.
C
pin, as shown in Figure 2. Generally a capacitor (C
LT3694
CURRENT MODE
POWER STAGE
g
C
m
F
= 7.5S
V
R
C
C
3M
C
F
Figure 2. Model for Loop Response
C
) is not part of the loop compensation but
C
) in series to ground are used. In addi-
350μS
g
AMPLIFIER
m
ERROR
=
–
+
GND
0.75V
SW
FB
R2
R1
TANTALUM
POLYMER
C
PL
ESR
C1
PL
OR
+
) is used or if
36941 F02
OUTPUT
C1
CERAMIC
36941fa
C
)