LTC3452 Datasheet, PDF (10/16 Pages) Linear Technology – Synchronous Buck-Boost MAIN/CAMERA White LED Driver

LTC3452
U
OPERATIO
LED Current Setting Amps
The maximum forward current per LED for all LEDs in a
given bank is programmed by a single external resistor to
ground at the corresponding I_SETL,H pin according to the
following formulas:
I_MAXL
=
256^{⎛⎝⎜}
0.8
R_ISETL
⎞⎠⎟_,
I_MAXH
=
768^{⎛⎝⎜}
0.8
R_ISETH
⎞
⎠⎟
For operation at currents below I_MAXL in the low power
bank, refer to the Exponential Brightness Control section
and also to external circuit options given in the Applica-
tions Section. For operation at currents below I_MAXH in the
high power bank, refer only to the external circuit options
given in the Applications Section.
implemented results in “smoother” brightness and dim-
ming control as perceived by the human eye, which is
logarithmic in nature.
Table 1. Low Power Bank Brightness Control
EN_L DUTY CYCLE (% LOGIC HIGH) LED_Lx CURRENT
0% (Logic Low)
0 (Shutdown)
0% < Duty Cycle < 12.5%
12.5% < Duty Cycle < 25%
25% < Duty Cycle < 37.5%
37.5% < Duty Cycle < 50%
50% < Duty Cycle < 62.5%
62.5% < Duty Cycle < 75%
75% < Duty Cycle < 87.5%
87.5% < Duty Cycle ≤ 100%
1/128 • I_MAXL
1/64 • I_MAXL
1/32 • I_MAXL
1/16 • I_MAXL
1/8 • I_MAXL
1/4 • I_MAXL
1/2 • I_MAXL
I_MAXL
Shutdown Circuit
The shutdown circuit monitors the voltages at the EN_L,H
pins. Logic high on either/both inputs enables the part and
logic low on both puts the part in shutdown. Since the EN_L
pin doubles as a PWM input for LED brightness control, an
output filter in the shutdown circuit is employed to prevent
the part from toggling in and out of shutdown for normal
PWMing of the EN_L input when EN_H is low. If EN_H is low,
the LTC3452 is enabled immediately after a rising edge at
the EN_L pin, but waits 200µs (typical) after a falling edge
to enter shutdown. Consequently, a minimum PWM fre-
quency is required for smooth brightness control at cur-
rents below I_MAXL. The recommended PWM frequency is
10kHz to 50kHz.
Exponential Brightness Control
(Low Power LED Bank Only)
The LTC3452 implements an exponential brightness con-
trol function for the low power LED bank only in which the
LED_Lx current is a function of the PWM duty cycle at the
EN_L pin. The LED current will be equal to a fraction of
I_MAXL as given in Table 1. As the duty cycle (that the PWM
waveform is logic high) increases linearly, the LED cur-
rent will increase exponentially from 1/128th I_MAXL to
128/128ths I_MAXL in seven binary steps. The function
LED Current Sources
Each LED pin is driven by a current source specifically
designed for low dropout. The LTC3452 employs a propri-
etary architecture that determines which of the seven
LEDs requires the largest forward voltage drop at its
programmed current, and then generates a feedback
voltage based on this one for closing the buck-boost
regulation loop. This results in the lowest output voltage
required for regulating all of the LEDs and thus the highest
LED power efficiency. The voltage present at the LED pin
of the “controlling LED” will be typically 130mV at 20mA
(low power bank) or 250mA at 100mA (high power bank)
of current.
LED Detect Circuit
If fewer than five LED outputs in the low power bank and/
or fewer than two LED outputs in the high power bank are
required, unused outputs should be connected to V_OUT.
Each LED pin has an internal LED detect circuit that
disables the output current source to save power if an
output is not needed. A small current is employed to detect
the presence of an LED at startup. This current is typically
10µA for the low power bank and 30µA for the high power
bank.
3452f
10