lt3645 Linear Technology Corporation, lt3645 Datasheet - Page 12

• Manufacturer Part Number: lt3645
• Description: 36v 500ma Step-down Regulator And 200ma Ldo
• Manufacturer: Linear Technology Corporation

LT3645
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:
where f is the switching frequency in MHz and C
μF. This choice will provide low output ripple and good
transient response. C
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
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
0.1Ω 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
Murata
Taiyo Yuden
Vishay Siliconix
TDK
12
C
OUT
= 26.4/(V
OUT
• ƒ)
OUT
OUT
= 10μF is a good choice for
will have less than its nominal
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
OUT
is in
BOOST Pin Considerations
The external capacitor C2 and an internal Schottky diode
connected between the V
charge pump circuit which is used to generate a boost
voltage that is higher than the input voltage (V
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
V
external Schottky diode (such as a BAS70) is connected
from V
If no voltage supply greater than 2.6V is available, then
an external boost Schottky diode can be tied from the
V
Figure 3. In this confi guration, the BOOST capacitor will be
charged to approximately the V
if V
ing V
power switch Q1 turns on, V
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
CC2
IN
IN
pin (anode) to the BOOST pin (cathode) as shown in
may be tied to a supply between 2.2V and 2.6V if an
IN
changes. In this confi guration the maximum operat-
CC2
CC2
V
is 25V, because when V
IN
must be tied to a supply greater than 2.6V.
(anode) to BOOST (cathode).
V
MAX V
BOOST
BOOST
V
D2
– V
IN
SW
BOOST
LT3645
GND
2V
V
IN
IN
Figure 3.
CC2
SW
SW
IN
~ 25V, and since the boost
and BOOST pins form a
IN
voltage, and will change
= 25V, then when the
C3
3645 F03
IN
V
OUT
). In most
3645f