LM3406_09 Datasheet, PDF (8/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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_O-MAX determines how many LEDs can be
driven in series. Referring to the illustration in Figure 1, output
voltage is calculated as:
V_O-MAX = V_IN-MIN x (1 - f_SW x t_OFF-MIN)
t_OFF-MIN = 230 ns
Once V_O-MAX has been calculated, the maximum number of
series LEDs, n_MAX, can be calculated by the following espres-
sion and rounding down:
30020306
FIGURE 1. Comparator and One-Shot
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_ON, duty cycle D and switching frequency
f_SW are related by the following expression:
f_SW = D / t_ON
D = (V_O + V_D) / (V_IN - V_SW + V_D)
V_D = Schottky diode (typically 0.5V)
V_SW = I_F x R_DSON
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_SNS, which can be deter-
mined using the following simple expression due to the output
averaging:
R_SNS = 0.2 / I_F
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_O-MAX, that the LED driver can provide. A
n_MAX = V_O-MAX / V_F
V_F = forward voltage of each LED
At low switching frequency V_O-MAX is higher, allowing the
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 Figure
1, output voltage is calculated as:
V_O = n x V_F + V_SNS
V_SNS = sense voltage of 200 mV, n = number of LEDs in series
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, how-
ever it may be done if certain precautions are taken. Be sure
that the external supply will not violate any absolute maximum
condition and will at no point exceed the voltage applied to
the VIN pins.
INTERNAL MOSFET AND DRIVER
The LM3406/06HV features an internal power MOSFET as
well as a floating driver connected from the SW pin to the
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