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

LT3645IMSE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3645
10
3645f
FB Resistor Networks
T h e o u t p u t v o l t a g e s a r e p r o g r a m m e d w i t h r e s i s t o r d i v i d e r s
between the outputs and the V
FB
and V
FB2
pins. Choose
the resistors according to
R1" R2 t
V
OUT
0.8
–1
©
«
ª
¹
»
º
R3 " R4 t
V
OUT2
0.797
–1
©
«
ª
¹
»
º
R2 and R4 should be 20k or less to avoid bias current
errors. In the step-down converter, an optional phase
lead capacitor of 22pf between V
OUT
and V
FB
reduces
light-load ripple.
Input Voltage Range
The maximum operating input voltage for the LT3645 is
36V. The minimum input voltage is determined by either
the LT3645’s minimum operating voltage of 3.6V or by
its maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC = (V
OUT
+ V
D
)/(V
IN
– V
SW
+ V
D
)
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
= ((V
OUT
+ V
D
)/DC
MAX
) – V
D
+ V
SW
with DC
MAX
= 0.83 for the LT3645.
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BOOST pins. For fi xed
frequency operation, the maximum input voltage is de-
termined by the minimum duty cycle, which is:
V
IN(MAX)
= ((V
OUT
+ V
D
)/DC
MIN
) – V
D
+ V
SW
with DC
MIN
= 0.075 for the LT3645.
Note that this is a restriction on the operating input voltage
for continuous mode operation. The circuit will continue
to regulate the output up until the overvoltage lockout
input voltage (38.5V). The part will tolerate transient input
APPLICATIONS INFORMATION
voltages up to 55V, but once the input voltage exceeds
36V, the power switch will shut off and stop regulating
the output voltage until the input voltage falls below 36V.
Minimum On Time
The LT3645 will operate at the correct frequency while
the input voltage is below V
IN(MAX)
. At input voltages
that exceed V
IN(MAX)
, the LT3645 will still regulate the
output properly (up to 38.5V); however, the LT3645 will
skip pulses to regulate the output voltage resulting in
increased output voltage ripple.
Figure 1 illustrates switching waveforms for a LT3645
application with V
OUT
= 1.2V near V
IN(MAX)
= 21.3V.
Figure 1.
As the input voltage is increased, the part is required
to switch for shorter periods of time. Delays associated
with turning off the power switch dictate the minimum on
time of the part. The minimum on time for the LT3645 is
100ns. Figure 2 illustrates the switching waveforms when
the input voltage is increased to V
IN
= 22V.
3645 F01
SWITCH
VOLTAGE
10V/DIV
INDUCTOR
CURRENT
0.5A/DIV
V
IN
= 18V
V
OUT
= 1.2V
I
OUT
= 500mA
C
OUT
= 10µF
L = 10µH
3645 F02
SWITCH
VOLTAGE
10V/DIV
INDUCTOR
CURRENT
0.5A/DIV
V
IN
= 22V
V
OUT
= 1.2V
I
OUT
= 500mA
C
OUT
= 10µF
L = 10µH
Figure 2.
LT3645
11
3645f
APPLICATIONS INFORMATION
Now the required on time has decreased below the mini-
mum on time of 100ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the part skips a few pulses so that the aver-
age inductor current meets and does not exceed the load
current requirement.
The LT3645 is robust enough to survive prolonged opera-
tion under these conditions as long as the peak inductor
current does not exceed 1.2A. Inductor saturation due
to high current may further limit performance in this
operating region.
Inductor Selection and Maximum Output Current
Choose the inductor value according to:
L = 2.2 •(V
OUT
+ V
D
)/ƒ
where V
D
is the forward voltage drop of the catch diode
(~0.4V), f is the switching frequency in MHz and L is in
H. With this value, there will be no subharmonic oscilla-
tion for applications with 50% or greater duty cycle. For
robust operation in fault conditions, the saturation current
should be above 1.5A. To keep effi ciency high, the series
resistance (DCR) should be less than 0.1. Table 1 lists
several inductor vendors. If the buck load current is less
than 500mA, then a lower valued inductor can be used.
Catch Diode
Depending on load current, a 500mA to 1A Schottky diode
is recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
overvoltage lockout voltage (38.5V). The ON Semiconduc-
tor MBRA140T3 and Central Semiconductor CMMSH1-40
are good choices, as they are rated for 1A continuous
forward current and a maximum reverse voltage of 40V.
Input Filter Network
Bypass V
IN
with a 1F or higher ceramic capacitor of X7R
or X5R type. Y5V types have poor performance over tem-
perature and applied voltage and should not be used. A 1F
ceramic capacitor is adequate to bypass the LT3645 and
will easily handle the ripple current. However, if the input
power source has high impedance, or there is signifi cant
inductance due to long wires or cables, additional bulk
capacitance might be necessary. This can be provided
with a low performance (high ESR) electrolytic capacitor
in parallel with the ceramic device. Step-down regulators
draw current from the input supply in pulses with very
fast rise and fall times. The input capacitor is required to
reduce the resulting voltage ripple at the LT3645 input
and to force this very high frequency switching current
into a tight local loop, minimizing EMI. A 1F capacitor
is capable of this task, but only if it is placed close to the
LT3645 and catch diode (see the PCB layout section). A
second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3645.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (underdamped) tank cir-
cuit. If the LT3645 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3645’s voltage rating. This situation can
easily be avoided. For more de t ails, se e Linear Technology
Application Note 88.
Table 1. Inductor Vendors
Vendor URL Part Series Inductance Range (μH) Size (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH8D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko www.toko.com A916CY
D585LC
2 to 12
1.1 to 39
6.3 × 6.2
8.1 × 8.0
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
LT3645
12
3645f
APPLICATIONS INFORMATION
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated
by the LT3645 to produce the DC output. In this role it
determines the output ripple so low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT3645’s control loop.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is:
C
OUT
= 26.4/(V
OUT
• ƒ)
where f is the switching frequency in MHz and C
OUT
is in
µF. This choice will provide low output ripple and good
transient response. C
OUT
= 10µF is a good choice for
output voltages above 2.5V. For lower output voltages
use 22µF or higher.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor R1 may be required to get the full benefi t (see the
Compensation section). Using a small output capacitor
results in an increased loop crossover frequency.
Use X5R or X7R types and keep in mind that a ceramic
capacitor biased with V
OUT
will have less than its nominal
capacitance. High performance electrolytic capacitors can
be used for the output capacitor. Low ESR is important, so
choose one that is intended for use in switching regulators.
The ESR should be specifi ed by the supplier and should be
0.1 or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR.
Table 2 lists several capacitor vendors.
Table 2. Capacitor Vendors
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
BOOST Pin Considerations
The external capacitor C2 and an internal Schottky diode
connected between the V
CC2
and BOOST pins form a
charge pump circuit which is used to generate a boost
voltage that is higher than the input voltage (V
IN
). In most
application circuits where the duty cycle is less than 50%,
use C2 = 0.1µF. If the duty cycle is higher than 50% then
use C2 = 0.22µF.
The BOOST pin must be at least 2.2V above the SW pin
to fully saturate the NPN power switch (Q1). The forward
drop of the internal Schottky diode is 0.8V. This means
that V
CC2
must be tied to a supply greater than 2.6V.
V
CC2
may be tied to a supply between 2.2V and 2.6V if an
external Schottky diode (such as a BAS70) is connected
from V
CC2
(anode) to BOOST (cathode).
If no voltage supply greater than 2.6V is available, then
an external boost Schottky diode can be tied from the
V
IN
pin (anode) to the BOOST pin (cathode) as shown in
Figure 3. In this confi guration, the BOOST capacitor will be
charged to approximately the V
IN
voltage, and will change
if V
IN
changes. In this confi guration the maximum operat-
ing V
IN
is 25V, because when V
IN
= 25V, then when the
power switch Q1 turns on, V
SW
~ 25V, and since the boost
capacitor is charged to 25V, the BOOST pin will be at 50V.
This connection is not as effi cient as the others because
the BOOST pin current comes from a higher voltage.
The minimum operating voltage of an LT3645 application
is limited by the undervoltage lockout (~3.4V) and by
the maximum duty cycle as outlined above. For proper
startup, the minimum input voltage is also limited by the
Figure 3.
V
IN
BOOST
GND
SW
V
IN
LT3645
D2
3645 F03
V
OUT
C3
V
BOOST
– V
SW
% V
IN
MAX V
BOOST
% 2V
IN
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24

LT3645IMSE#TRPBF

Mfr. #:
Buy LT3645IMSE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36Vin (55V Trans) 500mA Buck plus 200mA LDO
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet