LM3406HVEVAL National Semiconductor, LM3406HVEVAL Datasheet - Page 8

• Manufacturer Part Number: LM3406HVEVAL
• Description: LM3406/06HV 1.5A Constant Current Buck Regulator for Driving High Power LEDs; Qty per Container: 1/
• Manufacturer: National Semiconductor

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SWITCHING FREQUENCY
The LM3406/06HV does not contain a clock, however the on-
time is modulated in proportion to both input voltage and
output voltage in order to maintain a relatively constant fre-
quency. On-time t
f
The LM3406/06HV regulators should be operated in contin-
uous conduction mode (CCM), where inductor current stays
positive throughout the switching cycle. During steady-state
CCM operation, the converter maintains a constant switching
frequency that can be estimated using the following equation
(for the most accurate version, particularly for applications
that will have an input or output voltage of less than approxi-
mately 12V, see the Appendix):
SETTING LED CURRENT
LED current is set by the resistor R
mined using the following simple expression due to the output
averaging:
MAXIMUM NUMBER OF SERIES LEDS
LED driver designers often want to determine the highest
number of LEDs that can be driven by their circuits. The limit
on the maximum number of series LEDs is set by the highest
output voltage, V
SW
are related by the following expression:
FIGURE 1. Comparator and One-Shot
V
D
D = (V
= Schottky diode (typically 0.5V)
O-MAX
ON
, duty cycle D and switching frequency
O
V
+ V
R
, that the LED driver can provide. A
SW
f
SW
SNS
D
= I
) / (V
= D / t
= 0.2 / I
F
x R
IN
ON
DSON
- V
SNS
F
SW
, which can be deter-
+ V
D
)
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buck regulator cannot provide an output voltage that is higher
than the minimum input voltage, and in pratice the maximum
output voltage of the LM3406/06HV is limited by the minimum
off-time as well. V
driven in series. Referring to the illustration in
voltage is calculated as:
t
Once V
series LEDs, n
sion and rounding down:
V
At low switching frequency V
LM3406/06HV to regulate output voltages that are nearly
equal to input voltage, and this can allow the system to drive
more LEDs in series. Low switching frequencies are not al-
ways desireable, however, because they require larger, more
expensive components.
CALCULATING OUTPUT VOLTAGE
Even though output current is the controlled parameter in LED
drivers, output voltage must still be calculated in order to de-
sign the complete circuit. Referring to the illustration in
1, output voltage is calculated as:
V
MINIMUM ON-TIME
The minimum on-time for the LM3406/06HV is 280 ns (typi-
cal). One practical example of reaching the minimum on-time
is when dimming the LED light output with a power FET
placed in parallel to the LEDs. When the FET is on, the output
voltage drops to 200 mV. This results in a small duty cycle
and in most circuits requires an on-time that would be less
than 280 ns. In such a case the LM3406/06HV keeps the on-
time at 280 ns and increases the off-time as much as needed,
which effectively reduces the switching frequency.
HIGH VOLTAGE BIAS REGULATOR (VCC)
The LM3406/06HV contains an internal linear regulator with
a 7V output, connected between the VIN and the VCC pins.
The VCC pin should be bypassed to the GND pin with a 0.1
µF ceramic capacitor connected as close as possible to the
pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical)
and then regulates at 7V as VIN increases. The
LM3406/06HV comes out of UVLO and begins operating
when VCC crosses 5.3V. This is shown graphically in the
Typical Performance curves.
Connecting an external supply to VCC to power the gate
drivers is not recommended. However, it may be done if cer-
tain precautions are taken. Be sure that the external supply
will not violate any absolute maximum conditions and will at
no point exceed the voltage applied to the VIN pins. Under
certain conditions, some ringing may be present on the SW
and BOOT pins when VCC is driven with an external supply.
It is important to ensure that the absolute maximum ratings of
OFF-MIN
F
SNS
= forward voltage of each LED
= sense voltage of 200 mV, n = number of LEDs in series
O-MAX
= 230 ns
V
O-MAX
has been calculated, the maximum number of
MAX
, can be calculated by the following espres-
O-MAX
= V
V
n
MAX
O
IN-MIN
= n x V
determines how many LEDs can be
= V
x (1 - f
O-MAX
O-MAX
F
+ V
SW
/ V
SNS
is higher, allowing the
x t
F
OFF-MIN
Figure
)
1, output
Figure