LTM8040IV#PBF Linear Technology, LTM8040IV#PBF Datasheet - Page 12

• Manufacturer Part Number: LTM8040IV#PBF
• Description: IC, LED DRIVER/CURRENT SOURCE, LGA-66
• Manufacturer: Linear Technology

Specifications of LTM8040IV#PBF

• Led Driver Application: Automotive, Avionic Lighting
• No. Of Outputs: 1
• Output Current: 1A
• Output Voltage: 13V
• Input Voltage: 4V To 36V
• Dimming Control Type: PWM
• Topology: Buck
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

LTM8040
APPLICATION INFORMATION
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
be calculated from the maximum PWM period (t
and minimum PWM pulse width (t
Total dimming ratio (DIM
dimming ratio and the current dimming ratio.
12
t
t
MAX
MIN
= PWM
RATIO
RATIO
) is the product of the PWM
MIN
) as follows:
60Hz TO
Figure 5. Dimming Using PWM Signal
10kHz
PWM
RATIO
) can
MAX
)
Example: I
t
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
details.
MIN
PWM
LEDA
I
PWM
DIM
RATIO
LTM8040
= 40μs
GND
RATIO
IN
RATIO
=
8040 F05
). See Typical Performance Characteristics for
0.1A
MAX
1A
=10 • 250 = 2500:1
=
10ms
=10:1
40µs
= 1A, I
= 250:1
MIN
= 0.1A, t
MAX
= 10ms,
8040fa