ltm8040 Linear Technology Corporation, ltm8040 Datasheet - Page 10

• Manufacturer Part Number: ltm8040
• Description: 36v, 1a ?module Led Driver
• Manufacturer: Linear Technology Corporation

LTM8040
APPLICATION INFORMATION
cision 1.25V voltage source through a 5.11K 1% resistor.
This resistor makes it easy to adjust the LED current with
a single external resistor. For a 1A LED current, leave the
ADJ pin fl oating. For lower output currents, apply a re-
sistor from ADJ to GND as shown in Figure 3, using the
following formula:
R
where R
of LEDA.
In order to have accurate LED current, a precision 1%
resistor is recommended.
The LEDA voltage must be at least 2.4V for proper current
regulation.
Dimming Control
There are several different types of dimming control
circuits. One dimming control circuit (Figure 4) changes
the voltage on the ADJ pin by tying a low on-resis-
tance FET to the resistor divider string. This allows the
selection of two different LED currents. For reliable
operation, program an LED current of no less than
35mA. The maximum current dimming ratio (IRATIO)
10
ADJ
= 5.11 • I
ADJ
Figure 4. Dimming with an NFET and Resistor
Figure 3. Setting ADJ with a Resistor
DIM
is in kΩ and I
LED
/(1Amp – I
R
ADJ
R2
REF
ADJ
REF
V
LED
ADJ
LTM8040
LTM8040
GND
LED
GND
is the desired current out
),
8040 F03
8040 F04
can be calculated from the maximum LED current
(I
Another dimming control circuit (Figure 5) uses the PWM
pin and an external NFET tied to the cathode of the LED.
When the PWM signal goes low, the NFET turns off, dis-
connecting the LED from the internal current source and
“freezing” the state of LTM8040 internal control and drive
circuitry, but leaving the output capacitor of the internal
step down converter charged. When the PWM pin goes
high again, the LED current returns rapidly to its previous
on state. This fast settling time allows the LTM8040 to
maintain LED current regulation with PWM pulse widths
as short as 40μs. It is also possible to not use an external
NFET, but the output capacitor of the internal regulator
will be discharged by the LED(s) and have to be charged
up again when the current source turns back on. This
will lengthen the minimum dimming pulse width, in turn
reducing the PWM dimming frequency.
The maximum PWM dimming ratio (PWM
be calculated from the maximum PWM period (t
and minimum PWM pulse width (t
MAX
I
I
t
MAX
t
MIN
MAX
MIN
) and the minimum LED current (I
=I
= PWM
60Hz TO
RATIO
10kHz
Figure 5. Dimming Using PWM Signal
PWM
RATIO
PWM
LEDA
LTM8040
GND
8040 F05
MIN
MIN
) as follows:
) as follows:
RATIO
) can
MAX
8040p
)