LTC3452_15 Datasheet, PDF (12/16 Pages) Linear Technology – Synchronous Buck-Boost MAIN/CAMERA White LED Driver

LTC3452
APPLICATIO S I FOR ATIO
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:
f_{FILTER_POLE} = _{2 • π •}
1
Hz
L • C_OUT
where C_OUT is the output filter capacitor.
The output filter zero is given by:
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
f_UG
=
2
•
g_m
π • C_VC
where g_m is the error amp transconductance (typically
1/5.2k) and C_VC is the external capacitor to GND at the
V_C pin. For the white LED application, a 0.1µF or greater
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:
f_{FILTER_ ZERO}
=
2
•
π
1
• R_ESR
•
C_OUT
Hz
where R_ESR is the capacitor equivalent series resistance.
A troublesome feature in Boost mode is the right-half
plane zero (RHP), and is given by:
f_RHPZ
=
V_IN2
2 • π •I_OUT • L• V_OUT
Hz
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.
I_MAXL
=
256^{⎛⎝⎜}
0.8V
R_ISETL
⎞
⎠⎟
and
I_MAXH
=
768^{⎛⎝⎜}
0.8V
R_ISETH
⎞
⎠⎟
Since the maximum LED current for the low power bank is
25mA, this sets a minimum limit on R_ISETL of:
R_MINL
=
256
⎛
⎝⎜
0.8V
25mA
⎞
⎠⎟
=
8192Ω
Similarly, for the high power bank:
R_MINH
=
768
⎛
⎝⎜
0.8V
150mA
⎞
⎠⎟
=
4096Ω
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.
3452f
12