LM3464MH National Semiconductor, LM3464MH Datasheet - Page 15

• Manufacturer Part Number: LM3464MH
• Description: IC, LED DRIVER, BUCK, 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3464MH

• No. Of Outputs: 4
• Input Voltage: 12V To 80V
• Driver Case Style: TSSOP
• Svhc: No SVHC (15-Dec-2010)
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Topology: Linear
• Efficiency: 98%
• Led Driver Application: Streetlights, Solid State Lighting Solutions
• Dimming Control Type: PWM / Analogue
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

To define the minimum thermal dimming duty cycle, V
should be set in between 0.4V to 3.25V. The minimum duty
cycle is governed by the following equation:
When V
mum thermal dimming duty cycle limit is disabled. In applica-
LOW POWER STANDBY
The LM3464/64A will enter low power standby mode when
the EN pin is pulled to GND. The EN pin is internally biased
thus no external pull-up resistor or bias is required. Under
standby mode, all the output channels are cut-off and part of
the internal circuitries are disabled to maintain low power
consumption. Upon the EN pin is pulled low, the OutP pin
stopa sinking current from the feedback node of the primary
power stage. This causes the rail voltage fall back to V
(nom)
supply are being discharged by the LEDs. Pulling the EN pin
low will not disable the V
released (floating), the LM3464/64A exits low power standby
FIGURE 10. Thermal Foldback Control with Minimum
slowly as the output capacitors of the primary power
DMIN
is below 0.4V (e.g. connect to GND), the mini-
D
MINIMUM
Dimming Duty Cycle Limit
for 0.4
= [(V
≤
CC
V
DMIN
DMIN
regulator. When the EN pin is
- 0.4) X 35]%
≤
FIGURE 11. Thermal Foldback + PWM Dimming Control
3.25V
30115042
DMIN
RAIL
15
tions that thermal foldback control is not required, the DMIN
pin can be tied to GND to reduce power consumption.
PWM DIMMING
The LM3464/64A provides a DIM pin that accepts TTL logic
level signal for PWM dimming. When the DIM pin is pulled
low, all LED current regulators will turn off while maintaining
V
External pull up resistor is required if the DIM pin is driven by
an open collector / drain driver. PWM dimming guarantees
uniform color temperature of the light throughout the entire
dimming range. The average current of every output channel
is decided by the dimming duty cycle and follows the equation
below:
PWM DIMMING CONTROL WITH THERMAL FOLDBACK
The PWM dimming control can coexist with thermal foldback
by applying PWM dimming control signal and thermal control
signal to the DIM and Thermal pins concurrently. Normally,
the dimming frequency for thermal foldback control should be
much higher than the frequency of the PWM dimming control
signal. Figure 11 presents the relationship among V
V
thermal foldback is functioning, the average output current
can be further decreased linearly according to the duty cycle
of the PWM dimming signal being applied to the DIM pin. In
order to synchronize the dimming signals, the C
charged on every rising edge of the PWM dimming signal on
DIM pin, notice as t
mode and the startup sequence begins as described in Figure
3.
FAULT HANDLING and INDICATION
The LM3464/64A features a complete mechanism for fault
handling and indication. The LM3464/64A detects LED fail-
ures and raises fault indication signal at the Faultb pin upon
open or short circuits of LED strings, insufficient supply volt-
age and so on. In order to avoid false triggering the fault
detection circuitry, the LM3464/64A features a timer for fault
recognition. When a fault condition arises and sustains longer
than the time constant preset by the capacitor , C
is confirmed. The Faultb pin is then pulled low as an indica-
tion. The time constant for fault detection is defined by the
CC
Thermal_Cap
regulator and part of the internal circuitries operating.
, V
DIM
and I
I
LED(AVG)
1
, t
2
and t
LED
. As shown in the Figure, when
= D
3
in Figure 11.
PWM
x I
LED
www.national.com
THM
FLT
, a fault
Thermal
is dis-
30115035
,