Datasheet

LTM8040
12
8040fa
Example: I
MAX
= 1A, I
MIN
= 0.1A, t
MAX
= 10ms,
t
MIN
= 40μs
I
RATIO
=
1A
0.1A
= 10:1
PWM
RATIO
=
10ms
40µs
= 250:1
DIM
RATIO
= 10 • 250 = 2500:1
Minimum Input Voltage
The LTM8040 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. For most applications at full load, the input
needs to be at least 1.5V above the desired output. In
addition, it takes more input voltage to initially start than
is required for continuous operation. This start voltage is
also dependent on whether turn-on is controlled by the
LTM8040’s SHDN pin or UVLO (that is, the SHDN pin is
tied to V
IN
). See Typical Performance Characteristics for
details.
PWM dimming control (Figure 5) uses the PWM pin and
an ex ternal NFET tied to the cathode of the LED. When the
PWM signal goes low, the NFET turns off, disconnecting
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.
The maximum PWM dimming ratio (PWM
RATIO
) can
be calculated from the maximum PWM period (t
MAX
)
and minimum PWM pulse width (t
MIN
) as follows:
t
MAX
t
MIN
= PWM
RATIO
Total dimming ratio (DIM
RATIO
) is the product of the PWM
dimming ratio and the current dimming ratio.
APPLICATION INFORMATION
Figure 5. Dimming Using PWM Signal
GND
LTM8040
8040 F05
PWM
PWM
60Hz TO
10kHz
LEDA