OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT3580IDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
LT3580
10
3580fg
APPLICATIONS INFORMATION
these materials retain their capacitance over wider voltage
and temperature ranges. A 4.7μF to 20μF output capaci-
tor is sufficient for most applications, but systems with
very low output currents may need only a 1μF or 2.2μF
output capacitor. Always use a capacitor with a sufficient
voltage rating. Many capacitors rated at 2.2μF to 20μF,
particularly 0805 or 0603 case sizes, have greatly reduced
capacitance at the desired output voltage. Solid tantalum
or OS-CON capacitors can be used, but they will occupy
more board area than a ceramic and will have a higher
ESR with greater output ripple.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as closely as
possible to the LT3580. A 2.2μF to 4.7μF input capacitor
is sufficient for most applications.
Table 2 shows a list of several ceramic capacitor manufac-
turers. Consult the manufacturers for detailed information
on their entire selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Compensation—Adjustment
To compensate the feedback loop of the LT3580, a series
resistor-capacitor network in parallel with a single capacitor
should be connected from the VC pin to GND. For most
applications, the series capacitor should be in the range
of 470pF to 2.2nF with 1nF being a good starting value.
The parallel capacitor should range in value from 10pF to
100pF with 47pF a good starting value. The compensation
resistor, R
C
, is usually in the range of 5k to 50k. A good
technique to compensate a new application is to use a
100kΩ potentiometer in place of series resistor R
C
. With
the series capacitor and parallel capacitor at 1nF and 47pF
respectively, adjust the potentiometer while observing the
transient response and the optimum value for R
C
can be
found. Figures 3a to 3c illustrate this process for the circuit
of Figure 14 with a load current stepped between 400mA
and 500mA. Figure 3a shows the transient response with
R
C
equal to 1k. The phase margin is poor, as evidenced by
the excessive ringing in the output voltage and inductor
current. In Figure 3b, the value of R
C
is increased to 3k,
which results in a more damped response. Figure 3c
shows the results when R
C
is increased further to 10k. The
transient response is nicely damped and the compensation
procedure is complete.
Figure 3a. Transient Response Shows Excessive Ringing Figure 3b. Transient Response Is Better
Figure 3c. Transient Response Is Well Damped
V
OUT
200mV/DIV
AC COUPLED
I
L
0.5A/DIV
200μs/DIVR
C
= 1k
3580 F03a
V
OUT
200mV/DIV
AC COUPLED
I
L
0.5A/DIV
200μs/DIVR
C
= 3k
3580 F03b
V
OUT
200mV/DIV
AC COUPLED
I
L
0.5A/DIV
200μs/DIVR
C
= 10k
3580 F03c
LT3580
11
3580fg
APPLICATIONS INFORMATION
Compensation—Theory
Like all other current mode switching regulators, the
LT3580 needs to be compensated for stable and efficient
operation. Two feedback loops are used in the LT3580—
a fast current loop which does not require compensation,
and a slower voltage loop which does. Standard bode plot
analysis can be used to understand and adjust the voltage
feedback loop.
As with any feedback loop, identifying the gain and phase
contribution of the various elements in the loop is critical.
Figure 4 shows the key equivalent elements of a boost
converter. Because of the fast current control loop, the
power stage of the IC, inductor and diode have been replaced
by a combination of the equivalent transconductance
amplifier g
mp
and the current controlled current source
(which converts I
VIN
to ηV
IN
/V
OUT
• I
VIN
). g
mp
acts as
a current source where the peak input current, I
VIN
, is
proportional to the VC voltage. η is the efficiency of the
switching regulator, and is typically about 88%.
Note that the maximum output currents of g
mp
and g
ma
are
finite. The limits for g
mp
are in the Electrical Characteristics
section (switch current limit), and g
ma
is nominally limited
to about ±12μA.
From Figure 4, the DC gain, poles and zeros can be cal-
culated as follows:
Output Pole: P1=
2
2•π •R
L
•C
OUT
Error Amp Pole: P2 =
1
2•π •R
O
+R
C
⎡
⎣
⎤
⎦
•C
C
Error Amp Zero: Z1=
1
2•π •R
C
•C
C
DC Gain:
(Breaking Loop at FB Pin)
A
DC
= A
OL
(0) =
∂V
C
∂V
FB
•
∂I
VIN
∂V
C
•
∂V
OUT
∂I
VIN
•
∂V
FB
∂V
OUT
=
g
ma
•R
0
(
)
•g
mp
• η•
V
IN
V
OUT
•
R
L
2
⎛
⎝
⎜
⎞
⎠
⎟
•
0.5R2
R1+ 0.5R2
ESR Zero: Z2 =
1
2•π •R
ESR
•C
OUT
RHP Zero: Z3 =
V
IN
2
•R
L
2•π •V
OUT
2
•L
High Frequency Pole: P3>
f
S
3
Phase Lead Zero: Z4 =
1
2•π •R1•C
PL
Phase Lead Pole: P4 =
1
2•π •
R1•
R2
2
R1+
R2
2
•C
PL
Error Amp Filter Pole:
P5 =
1
2•π•
R
C
•R
O
R
C
+R
O
•C
F
,C
F
<
C
C
10
The current mode zero (Z3) is a right-half plane zero
which can be an issue in feedback control design, but is
manageable with proper external component selection.
Figure 4. Boost Converter Equivalent Model
–
+
–
+
g
ma
R
C
R
O
R2
R2
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
C
F
: HIGH FREQUENCY FILTER CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
DIVIDED BY I
LOAD(MAX)
R
O
: OUTPUT RESISTANCE OF g
ma
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
3580 F04
R1
FB
C
OUT
C
PL
R
L
R
ESR
V
OUT
I
VIN
V
C
C
C
C
F
g
mp
1.215V
REFERENCE
H•V
IN
V
OUT
•I
VIN
LT3580
12
3580fg
APPLICATIONS INFORMATION
Using the circuit in Figure 14 as an example, Table 3 shows
the parameters used to generate the bode plot shown in
Figure 5.
Table 3. Bode Plot Parameters
PARAMETER VALUE UNITS COMMENT
R
L
21.8
Ω
Application Specific
C
OUT
10 μF Application Specific
R
ESR
10
mΩ
Application Specific
R
O
305
kΩ
Not Adjustable
C
C
1000 pF Adjustable
C
F
0 pF Optional/Adjustable
C
PL
0 pF Optional/Adjustable
R
C
10
kΩ
Adjustable
R1 130
kΩ
Adjustable
R2 14.6
kΩ
Not Adjustable
V
OUT
12 V Application Specific
V
IN
5 V Application Specific
g
ma
230 μmho Not Adjustable
g
mp
7 mho Not Adjustable
L 4.2 μH Application Specific
f
S
1.2 MHz Adjustable
In Figure 5, the phase is –140° when the gain reaches 0dB
giving a phase margin of 40°. The crossover frequency
is 10kHz, which is more than three times lower than the
frequency of the RHP zero to achieve adequate phase
margin.
Diode Selection
Schottky diodes, with their low forward voltage drops and
fast switching speeds, are recommended for use with the
LT3580. The Microsemi UPS120 is a very good choice.
Where the input-to-output voltage differential exceeds 20V,
use the UPS140 (a 40V diode). These diodes are rated to
handle an average forward current of 1A.
Oscillator
The operating frequency of the LT3580 can be set by the
internal free-running oscillator. When the SYNC pin is
driven low (< 0.4V), the frequency of operation is set by
a resistor from R
T
to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency is
calculated using the following formula:
f
OSC
=
91.9
(R
T
+1)
where f
OSC
is in MHz and R
T
is in kΩ. Conversely, R
T
(in kΩ) can be calculated from the desired frequency
(in MHz) using:
R
T
=
91.9
f
OSC
−1
Clock Synchronization
The operating frequency of the LT3580 can be synchronized
to an external clock source. To synchronize to the external
source, simply provide a digital clock signal into the SYNC
pin. The LT3580 will operate at the SYNC clock frequency.
The LT3580 will revert to the internal free-running oscillator
clock after SYNC is driven low for a few free-running clock
periods.
Driving SYNC high for an extended period of time effectively
stops the operating clock and prevents latch SR1 from
becoming set (see the Block Diagram). As a result, the
switching operation of the LT3580 will stop.
The duty cycle of the SYNC signal must be between 35%
and 65% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
Figure 5. Bode Plot for Example Boost Converter
FREQUENCY (Hz)
10
60
GAIN (dB)
PHASE (DEG)
80
100
120
140
100 1k 10k 100k 1M
3580 F05
40
20
0
–20
160
180
–120
–100
–80
–60
–40
–140
–160
–180
–200
–20
0
40o AT
10kHz
PHASE
GAIN
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28

LT3580IDD#PBF

Mfr. #:
Buy LT3580IDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2A Boost/Inverting Switching Regulator
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet