Datasheet

LT3682
16
3682f
APPLICATIONS INFORMATION
Figure 2. Model for Loop Response
0.8V
LT3682
C
F
R
C
V
C
3Meg
3682 F02
CURRENT MODE
POWER STAGE
g
m
= 1.25S
C
C
C
PL
C1
R2
R1
SW
FB
GND
OUTPUT
CERAMIC
POLYMER
OR
TANTALUM
OR
ELECTROLITIC
ESR
+
C1
–
+
g
m
= 420μS
Table 3. Schottky Diodes
PART NUMBER V
R
(V) I
AVE
(A) V
F
at 1A (mV) V
F
at 2A (mV)
On Semiconducor
MBR0520L 20 0.5
MBR0540 40 0.5 620
MBRM120E 20 1 530 595
MBRM140 40 1 550
Diodes Inc.
B0530W 30 0.5
B0540W 40 0.5 620
B120 20 1 500
B130 30 1 500
B140 40 1 500
B150 50 1 700
B220 20 2 500
B230 30 2 500
B140HB 40 1
DFLS240L 40 2 500
DFLS140 40 1.1 510
B240 40 2 500
Central Semiconductor
CMSH1 – 40M 40 1 500
CMSH1 – 60M 60 1 700
CMSH1 – 40ML 40 1 400
CMSH2 – 40M 40 2 550
CMSH2 – 60M 60 2 700
CMSH2 – 40L 40 2 400
CMSH2 – 40 40 2 500
CMSH2 – 60 60 2 700
Frequency Compensation
The LT3682 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular,
the LT3682 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
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 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 LT3682 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
current proportional to the voltage at the V
C
pin. Note that
the output capacitor integrates this current, and that the
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 addition, there
may be a lower value capacitor in parallel. This capacitor
(C
F
) is used to fi lter noise at the switching frequency, and
is required only if a phase-lead capacitor (C
PL
) is used or
if the output capacitor has high ESR.