Analog Devices EVAL-LT8355-1-AZ, Руководство пользователя

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5
DEMO MANUAL EVAL-LT8355-1-AZ
Rev. 0
Full-scale LED current sense threshold voltage
(V
ISP
to V
ISN
) for LT8355-1 is 250mV, which sets full-
scale LED current according to Equation 1.
I
LED
= 250mV
R
SENSE
(1)
The LED current sense threshold voltage can be trimmed
to be lower than 250mV when analog dimming is desired.
This includes but is not limited to, when a single sense
resistor is used for multiple designs with different current
levels needed for each design, and when low current level
is required at start-up and increases at steady state, etc.
For configurations where the LED current sense threshold
(V
ISP
to V
ISN
) is configured for 75mV or lower via CTRL
or IADJ pin, an additional resistor connecting V
REF
to FB
is needed to allow for proper start-up of the LT8355-1
(see Figure 3). Refer to data sheet for more information.
See Equation 2 and Equation 3 to set resistor values for
boost and buck-boost mode topologies.
START-UP WITH LOW V
ISP
TO V
ISN
V
OUT
eval-lt8355-1-az F03
V
REF
V
OUT
V
REF
V
IN
FB FB
R1 R2 40k
10k
10k
10k
R1
Q
(a) Boost (a) Buck-Boost
Figure 3. Proper Star-Up Configuration
Boost: Calculate R1 and R2 to set desired V
OUT_OVP
volt-
age and V
FB
to 400mV at V
IN_MIN
.
V
IN_MIN
=
400mV
+
40µA
– 1.6V
R1
R2
V
OUT_OVP
=
1.2V
+
120µA
– 800mV
R1
R2
(2)
Buck-Boost: Calculate R1 to set V
LED_OVP
(V
OUT
– V
IN
) to
desired OVP voltage.
V
LED_OVP BE
=
V
+
120µA • R1
(3)
EVAL-LT8355-1-AZ has channel 1 assembled as a boost
LED driver and channel 2 as a buck-boost mode LED
driver. With minor adjustments channel 2 can be reconfig-
ured as a boost LED driver. In buck-boost mode, the LED–
connection is at V
IN
, and a level shifter is used for the FB
network. In a boost LED driver, the LED– connection is
at GND and a resistor divider is used for the FB network.
BOOST LED DRIVER SETUP
To configure EVAL-LT8355-1-AZ channel 2 as a boost
LED driver , remove R53, R52, Q2, FB10, C38 and C49.
Install 0Ω for R46 and 1M for R48. Consult the data sheet
for OVP calculations and details about the FB pin. Connect
the LED string from LED+ to LED– (GND boost).
Note that when EVAL-LT8355-1-AZ is reconfigured
as a boost LED driver, other components may need
to be adjusted depending on their voltage rating and
power capabilities.