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START-UP WITH LOW V
Full-scale LED current sense threshold voltage
(V
to V
) for LT8355-1 is 250mV, which sets full-
ISP
ISN
scale LED current according to Equation 1.
250mV
I
=
LED
R
SENSE
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
to V
) is configured for 75mV or lower via CTRL
ISP
ISN
or IADJ pin, an additional resistor connecting V
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.
BOOST LED DRIVER SETUP
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
, and a level shifter is used for the FB
IN
network. In a boost LED driver, the LED– connection is
at GND and a resistor divider is used for the FB network.
DEMO MANUAL EVAL-LT8355-1-AZ
TO V
ISP
ISN
(1)
Boost: Calculate R1 and R2 to set desired V
age and V
to FB
REF
Buck-Boost: Calculate R1 to set V
desired OVP voltage.
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.
V
V
REF
OUT
R1
R2
FB
FB
10k
(a) Boost
Figure 3. Proper Star-Up Configuration
to 400mV at V
FB
IN_MIN
= 400mV + 40µA –
V
IN_MIN
V
= 1.2V+ 120µA –
OUT_OVP
V
= V + 120µA • R1
LED_OVP
BE
V
OUT
R1
10k
V
Q
V
IN
REF
40k
10k
(a) Buck-Boost
eval-lt8355-1-az F03
volt-
OUT_OVP
.
1.6V
R2
R1
800mV
R2
R1
(V
– V
LED_OVP
OUT
IN
Rev. 0
5
(2)
) to
(3)
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