LTC3452 Linear Technology, LTC3452 Datasheet - Page 12

• Manufacturer Part Number: LTC3452
• Description: Synchronous Buck-Boost MAIN/CAMERA White LED Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3452
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 LTC3452 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 is required to be crossed over a
decade before the LC double pole.
12
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
U
2
2
V
• •
• •
IN
π
π
2
• •
L V
U
R
1
ESR
L C
1
OUT
•
•
OUT
C
Hz
OUT
W
Hz
Hz
U
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
where g
1/5.2k) and C
V
capacitor value is recommended.
Paralleling LED Outputs for Higher Current
Two or more LED output pins can be connected together
in parallel to achieve higher output current in fewer than 7
LEDs. For a very high power LED such as a LumiLED, all
7 outputs can be connected in parallel for maximum total
output current, as shown in the back page application of
this data sheet.
Maximum LED Current
As described in the Operation section, the maximum
output LED currents are equal to:
Since the maximum LED current for the low power bank is
25mA, this sets a minimum limit on R
Similarly, for the high power bank:
In addition, since the maximum continuous output current
for the buck-boost is limited to 425mA, this may impose
higher resistor value minimums if all outputs are used.
and
C
I
I
R
R
f
pin. For the white LED application, a 0.1µF or greater
MAXL
MAXH
UG
MINL
MINH
=
m
2 • •
=
=
=
=
is the error amp transconductance (typically
256
768
256
768
π
g
VC
m
⎛
⎜
⎝
⎛
⎜
⎝
C
⎛
⎝ ⎜
⎛
⎝ ⎜
R
R
is the external capacitor to GND at the
VC
25
150
0 8
0 8
0 8
ISETL
ISETH
0 8
.
.
.
mA
.
V
V
V
mA
V
⎞
⎟
⎠
⎞
⎠ ⎟
⎞
⎟
⎠
⎞
⎠ ⎟
=
=
8192
4096
Ω
w w w . D a t a S h e e t 4 U . c
Ω
ISETL
of:
3452f