MAX16807_07 Datasheet, PDF (15/21 Pages) Maxim Integrated Products – Integrated 8-Channel LED Drivers with Switch-Mode Boost and SEPIC Controller

Integrated 8-Channel LED Drivers with
Switch-Mode Boost and SEPIC Controller
where t_D is in seconds, R_T in ohms (Ω), and C_T in
Farads (F).
The oscillator frequency is then:
f_OSC
=
1
(^tC ⁺
t_D)
Reference Output
REF is a 5V reference output that can source 20mA.
Bypass REF to AGND with a 0.1µF capacitor.
Current Limit
The MAX16807/MAX16808 include a fast current-limit
comparator to terminate the on cycle during an over-
load or a fault condition. The current-sense resistor,
R_CS, connected between the source of the external
MOSFET and AGND, sets the current limit. The CS
input has a voltage trip level (V_CS) of 0.3V. Use the fol-
lowing equation to calculate R_CS:
R_CS
=
V_CS
I_{P − P}
I_P-P is the peak current that flows through the MOSFET.
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (OUT) turns the
switch off within 60ns. In most cases, a small RC filter is
required to filter out the leading-edge spike on the sense
waveform. Set the time constant of the RC filter at 50ns.
Buck-Boost (SEPIC) Operation
Figure 2 shows a buck-boost application circuit using
the MAX16807/MAX16808 in a stand-alone mode of
operation. SEPIC topology is necessary when the total
forward voltage of the LEDs in a string is such that
V_OUT can be below or above V_IN.
V_IN
L1
C1
D
V_OUT
C_OUT
Q
L2
R1
LEDs
R_CS
C_C2
C_C1
R_C1
R2
C_IN
3V TO 5.5V
C_BYP
EXTERNAL
CLOCK INPUT
EXTERNAL
DIM INPUT
R_SET
OUT CS AGND COMP
V_CC
FB
V+
MAX16807
PGND
MAX16808
OUT0
OUT1
DIN
OUT2
LE
OUT3
OUT4
CLK
OUT5
DOUT
OUT6
OE
OUT7
SET
REF
RTCT
R_T
C_REF
C_T
Figure 2. Buck-Boost (SEPIC) Configuration
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