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LTM8040
APPLICATION INFORMATION
I RATIO =
= 10:1
PWM RATIO =
= 250:1
= PWM RATIO
PWM dimming control (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, 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
Totaldimmingratio(DIM RATIO )istheproductofthePWM
dimming ratio and the current dimming ratio.
PWM
60Hz TO
10kHz
Example: I MAX = 1A, I MIN = 0.1A, t MAX = 10ms,
t MIN = 40μs
1A
0.1A
10ms
40μs
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
LTM8040
LEDA
GND
8040 F05
Figure 5. Dimming Using PWM Signal
8040fa
12
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