LTC3453_15 Datasheet, PDF (10/12 Pages) Linear Technology – Synchronous Buck-Boost High Power White LED Driver

LTC3453
APPLICATIO S I FOR ATIO
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 4
LEDs. For a very high power LED such as a LumiLED, all
four outputs can be connected in parallel for maximum
total output current, as shown in the cover page applica-
tion of this datasheet.
Maximum LED Current
As described in the Operation section, the output LED
current with both enable pins logic high is equal to
I_LED = 384 [0.8V/(R_ISET1 || R_ISET2)]
Since the maximum continuous output current is limited
to 500mA, this sets a minimum limit on the parallel
combination of R_ISET1 and R_ISET2 equal to
R_MIN = (R_ISET1 || R_ISET2)|_MIN = 4(384[0.8V/500mA])
= 2458Ω
Although the LTC3453 can safely provide this current
continuously, the external LED(s) may not be rated for this
high a level of continuous current. Higher current levels
are generally reserved for pulsed applications, such as
LED camera flash. This is accomplished by programming
a high current with one of the R_ISET resistors and pulsing
the appropriate enable pin.
Varying LED Brightness
Continuously variable LED brightness control can be
achieved by interfacing directly to one or both of the I_SET
pins. Figure 3 shows four such methods employing a
voltage DAC, a current DAC, a simple potentiometer or a
PWM input. It is not recommended to control brightness
by PWMing the enable pins directly as this will toggle the
LTC3453 in and out of shutdown and result in erratic
operation.
V_IN
V_OUT
V_IN
V_OUT
VOLTAGE
DAC
ENx
LED1
LTC3453
I_SETx
R_SET ≥ R_MIN
LED4
I_LED
=
384
0.8V – V_DAC
R_SET
V_DAC
(a)
V_IN
V_OUT
CURRENT
DAC
ENx
LED1
LTC3453
I_SETx
LED4
0.8V
IDAC ≤ _RMIN
I_LED = 384 • IDAC
(b)
V_IN
V_OUT
ENx
LED1
LTC3453
I_SETx
R_MIN
LED4
0.8V
I_LED = 384 _{RMIN + RPOT}
R_POT
(c)
R_SET
100
ENx
LED1
LTC3453
I_SETx
R_SET ≥ R_MIN
V_PWM
1µF
LED4
I_LED
=
384
0.8V – V_PWM
R_SET
=
384
0.8V
–
(DC% •
R_SET
V_DVCC)
DV_CC
f_PWM ≥ 5kHz
(d)
3453 F03
Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input
3453fa
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