LM3409MY National Semiconductor, LM3409MY Datasheet - Page 10

• Manufacturer Part Number: LM3409MY
• Description: LM3409 LM3409HV PFET Buck Controller for High Power LED Drives; Package: MINI SOIC EXP PAD; No of Pins: 10; Qty per Container: 1000/Reel
• Manufacturer: National Semiconductor

www.national.com
There are three different methods to set the current sense
threshold (V
1.
2.
3.
Controlled Off-Time
Once Q1 is turned off, it remains off for a constant time
(t
ternal capacitor (C
in Figure 3. Since I
nearly constant over widely varying input voltage and tem-
perature yielding a nearly constant t
At the start of t
and the capacitor begins charging according to the time con-
stant provided by R
off-time threshold (V
nated and v
follows:
In reality, there is typically 20 pF parasitic capacitance at the
off-timer pin in parallel with C
the calculation of t
equation has a preceding negative sign because the result of
the logarithm should be negative for a properly designed cir-
cuit. The resulting t
1.24V. If V
internally limited maximum off-time (typically 300µs) will oc-
cur.
OFF
IADJ pin left open: 5µA internal current source biases the
Zener diode and clamps the IADJ pin voltage (V
1.24V causing the maximum threshold voltage:
External voltage (V
pin to adjust V
is adjustable, analog dimming can be achieved.
External resistor (R
5µA current source sets the V
corresponding threshold voltage:
) which is preset by an external resistor (R
O
FIGURE 3. Off-Time Control Circuit
CST
< 1.24V, the off-timer cannot reach V
COFF
OFF
) using the multi-function IADJ pin:
(t) is reset to zero. t
, the voltage across C
OFF
OFF
CST
OFF
OFF
LED
OFT
), and the output voltage (V
. Also, it should be noted that the t
from 0V to 248mV. If the V
and C
ADJ
EXT
is a positive value as long as V
is tightly regulated, V
= 1.24V), then the off-time is termi-
) of 0V to 1.24V: Apply to the IADJ
) placed from IADJ pin to ground:
OFF
OFF
. When v
, which is accounted for in
ADJ
OFF
voltage and
.
OFF
OFF
COFF
(v
is calculated as
COFF
(t) reaches the
O
O
OFF
will remain
ADJ
) as shown
OFT
(t)) is zero
), an ex-
ADJ
voltage
and an
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) at
O
OFF
>
10
Although the t
linear in most applications. Ignoring the 20pF parasitic ca-
pacitance at the COFF pin, v
time derivative of v
approximation to the t
When t
the slope of the function is essentially linear and t
approximated as a current source charging C
Using the actual t
(Δi
Using the t
Δi
The Δi
L1, therefore the ripple is essentially constant over the oper-
ating range as long as V
mation is valid). An exception to the t
occurs if the IADJ pin is used to analog dim. As the LED/in-
ductor current decreases, the converter will eventually enter
DCM and the ripple will decrease with the peak current thresh-
old. The approximation shows how the LM3409/09HV
achieves constant ripple over a wide operating range, how-
ever t
presented.
L-PP
L-PP
FIGURE 4. Exponential Charging Function v
OFF
) of a buck current regulator operating in CCM is:
is independent of both V
L-PP
OFF
should be calculated using the actual equation first
<< R
OFF
approximation only depends on R
OFF
approximation, the equation is reduced to:
OFF
equation is non-linear, t
OFF
COFF
x C
OFF
OFF
equation, the inductor current ripple
(t) can be calculated to find a linear
O
equation:
(equivalent to when V
>> 1.24V (when the t
COFF
(t) is plotted in Figure 4. The
IN
and V
O
OFF
OFF
when in CCM!
OFF
is actually very
OFF
approximation
O
OFF
:
, C
OFF
>> 1.24V),
COFF
OFF
approxi-
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can be
(t)
, and