LTC3453 LINER [Linear Technology], LTC3453 Datasheet - Page 9

• Manufacturer Part Number: LTC3453
• Description: Synchronous Buck-Boost High Power White LED Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Input Capacitor Selection
Since the V
recommended to place at least a 2.2µF, low ESR bypass
capacitor to ground. See Table 2 for a list of component
suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER
AVX
Sanyo
Taiyo Yuden
TDK
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady
state ripple due to charge is given by:
where C
The output capacitance is usually many times larger in
order to handle the transient response of the converter.
For a rule of thumb, the ratio of the operating frequency to
the unity-gain bandwidth of the converter is the amount
the output capacitance will have to increase from the
above calculations in order to maintain the desired tran-
sient response.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden, TDK,
AVX ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. For the white
LED application, a 4.7µF capacitor value is recommended.
See Table 2 for a list of component suppliers.
%
%
Ripple Boost
Ripple Buck
OUT
_
_
IN
= output filter capacitor, F
pin is the supply voltage for the IC it is
=
=
8
U
I
(
OUT MAX
•
V
V
IN MAX
IN MAX
(
(
WEB SITE
www.avxcorp.com
www.sanyovideo.com
www.t-yuden.com
www.component.tdk.com
(
U
)
)
C C
)
•
–
OUT
•
(
V
V
f
OUT
2
OUT
•
• •
W
L C
V
)
OUT
–
•
1 1 00
V
OUT
IN MIN
2
(
•
f
%
)
U
)
•
100
%
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving efficiency. Use a Schottky
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifier diodes, since the slow recovery times
will compromise efficiency.
Closing the Feedback Loop
The LTC3453 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(Buck, Boost, Buck/Boost), but is usually no greater than
15. The output filter exhibits a double pole response
given by:
where C
The output filter zero is given by:
where R
A troublesome feature in Boost mode is the right-half
plane zero (RHP), and is given by:
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorpo-
rated to stabilize the loop but at a cost of reduced band-
width and slower transient response. To ensure proper
phase margin, the loop requires to be crossed over a
decade before the LC double pole.
f
f
f
FILTER POLE
FILTER ZERO
RHPZ
OUT
ESR
=
_
_
2 • •
is the output filter capacitor.
is the capacitor equivalent series resistance.
π
=
=
I
OUT
2
2
V
• •
• •
IN
π
π
2
• •
L V
R
1
L C
ESR
1
OUT
•
•
OUT
C
Hz
OUT
Hz
Hz
LTC3453
3453fa
9