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LT3507EUHF#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30
LT3507
13
3507fb
For more information www.linear.com/LT3507
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 LT3507 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.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Tantalum, as well as newer,
lower-ESR organic electrolytic capacitors intended for
power supply use are suitable. Chose a capacitor with a
low enough 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 2 lists
several capacitor vendors.
Table 2. Low ESR Surface Mount Capacitors
VENDOR TYPE SERIES
Taiyo-Yuden Ceramic
AVX Ceramic
Tantalum
TPS
Kemet Tantalum
T
antalum Organic
Aluminum Organic
T491,T494,T495
T520
A700
Sanyo T
antalum or
Aluminum Organic
POSCAP
Panasonic
Aluminum Organic SP CAP
TDK Ceramic
Diode Selection
The catch diode (D1 from Figure 2) 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
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current. Peak
reverse voltage is equal to the regulator input voltage. Use
a diode with a reverse voltage rating greater than the input
voltage, but not higher than 40V. Using higher breakdown
Schottky diodes may result in undesirable behavior. The
programmable OVLO can protect the diode from excessive
reverse voltage by shutting down the regulator if the input
voltage exceeds the maximum rating of the diode. Table
3 lists several Schottky diodes and their manufacturers.
Table 3. Schottky Diodes
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(mV)
V
F
AT 2A
(mV)
On Semiconductor
MBRM120E 20
1 530 595
MBRM140 40 1 550
Diodes Inc
B120 20 1 500
B140 40 1 500
B220 20 2 500
B240 40 2 500
DFLS140L 40 1 550
DFLS240L 40 2 550
Boost Pin Considerations
The capacitor and diode tied to the BOOST pin generate a
voltage that is higher than the input voltage. In most cases,
a small ceramic capacitor and fast switching diode (such
as the CMDSH-3 or MMSD914LT1) will work well. The
capacitor value is a function of the switching frequency,
applications inForMation
LT3507
14
3507fb
For more information www.linear.com/LT3507
peak current, duty cycle and boost voltage; in general a
value of (0.1µF • 1MHz/f
SW
) works well. Figure 3 shows
three ways to arrange the boost circuit. The BOOST pin
must be more than 2.5V above the SW pin for full ef
-
ficiency. For outputs of 3.3V and higher, the standard
circuit (Figure 3a) is best. For outputs between 2.8V and
3.3V, use a small Schottky diode (such as the BAT54).
For lower output voltages, the boost diode can be tied
to the input (Figure 3b). The circuit in Figure 3a is more
efficient because the BOOST pin current comes from a
lower voltage source. Finally, as shown in Figure 3c, the
anode of the boost diode can be tied to another source
that is at least 3V. For example, if you are generating 3.3V
and 1.8V and the 3.3V is on whenever the 1.8V is on, the
1.8V boost diode can be connected to the 3.3V output. In
this case, the 3.3V output cannot be set to track the 1.8V
output (see Output Voltage Tracking).
In any case, be sure that the maximum voltage at the
BOOST pin is less than 55V and the voltage difference
between the BOOST and SW pins is less than 25V.
The minimum operating voltage of an LT3507 applica
-
tion is limited by the internal undervoltage lockout (4V
for Channel 1, 3V
for Channels 2 and 3) and by the
maximum duty cycle. The boost circuit also limits the
minimum input voltage for proper start-up. If the input
voltage ramps slowly, or the LT3507 turns on when the
output is already in regulation, the boost capacitor may
not be fully charged. Because the boost capacitor charges
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
input and output voltages, and on the arrangement of
the boost circuit. The minimum load current generally
goes to zero once the circuit has started. Figure 4 shows
a plot of minimum load to start and to run as a function
of input voltage. Even without an output load current, in
many cases the discharged output capacitor will present
a load to the switcher that will allow it to start.
The boost current is generally small but can become sig
-
nificant at high duty cycles. The required boost current is:
I
BOOST
=
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
I
OUT
40
⎛
⎝
⎜
⎞
⎠
⎟
applications inForMation
Figure 3. Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT3507
(3a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT3507
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
3507 F03
D2
V
IN
BOOST
GND
SW
V
IN
LT3507
(3c)
V
OUT
V
BOOST
– V
SW
≅ V
INB
MAX V
BOOST
≅ V
INB
+ V
IN
MINIMUM VALUE FOR V
INB
=
3V
V
INB
> 3V
C3
LT3507
15
3507fb
For more information www.linear.com/LT3507
Converter with Backup Output Regulator
There is another situation to consider in systems where
the output will be held high when the input to the LT3507
is absent. If the V
IN
and one of the RUN pins are allowed
to float, then the LT3507’s internal circuitry will pull its
quiescent current through its SW pin. This is acceptable if
the system can tolerate a few mA of load in this state. With
all three RUN pins grounded, the LT3507 enters shutdown
mode and the SW pin current drops to <50µA. However, if
the V
IN
pin is grounded while the output is held high, then
parasitic diodes inside the LT3507 can pull large currents
from the output through the SW pin and the V
IN
pin. A
Schottky diode in series with the input to the LT3507, as
shown in Figure 5, will protect the LT3507 and the system
from a shorted or reversed input.
Input Capacitor Selection
Bypass the input of the LT3507 circuit with a 10µF or higher
ceramic capacitor of X7R or X5R type. A lower value or
a less expensive Y5V type will work if there is additional
bypassing provided by bulk electrolytic capacitors, or if the
input source impedance is low. The following paragraphs
describe the input capacitor considerations in more detail.
Step-down regulators draw current from the input sup
-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3507 input and to force this switching current
into a tight local loop, minimizing EMI. The input capaci
-
tor must have low impedance at the switching frequency
to do this effectively and it must have an adequate ripple
current rating.
With three switchers operating at the same
frequency but with different phases and duty cycles, cal
-
culating the input capacitor RMS current is not simple;
however,
a conservative value is the RMS input current
for the phase delivering the most power (V
OUT
• I
OUT
):
I
IN(RMS)
=I
OUT
•
V
OUT
V
IN
– V
OUT
( )
V
IN
<
I
OUT
2
applications inForMation
Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
5.5
5.0
4.0
3.0
4.5
3.5
2.5
3507 F04b
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 3.3V
TO START
TO RUN
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
8.0
7.5
6.5
5.5
4.5
7.0
6.0
5.0
4.0
3507 F04a
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 5V
TO START
TO RUN
Figure 5. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output
V
IN
V
IN
V
OUT
SW
LT3507
D4
PARASITIC DIODE
3507 F05
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LT3507EUHF#TRPBF

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Buy LT3507EUHF#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 3x Mono Buck Reg w/ LDO
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