LM3409MY-NOPB Datasheet, PDF (9/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

LM3409, LM3409HV, LM3409-Q1
www.ti.com
SNVS602J – MARCH 2009 – REVISED MAY 2013
The average inductor current (I_L) is equal to the average output LED current (I_LED), therefore if I_L is tightly
controlled, I_LED will be well regulated. As the system changes input voltage or output voltage, duty cycle (D) is
varied to regulate I_L and ultimately I_LED. For any buck regulator, D is simply the conversion ratio divided by the
efficiency (η):
D=
V_O
x V_IN
(1)
i_L (t)
I_L-MAX
üi_L-
I_L
PP
I_L-MIN
t_ON = DT_S t_OFF = (1-D)T_S
0
t
T_S
Figure 21. Ideal CCM Buck Converter Inductor Current i_L(t)
CONTROLLED OFF-TIME (COFT) ARCHITECTURE
The COFT architecture is used by the LM3409/09HV to control I_LED. It is a combination of peak current detection
and a one-shot off-timer that varies with output voltage. D is indirectly controlled by changes in both t_OFF and t_ON,
which vary depending on the operating point. This creates a variable switching frequency over the entire
operating range. This type of hysteretic control eliminates the need for control loop compensation necessary in
many switching regulators, simplifying the design process and providing fast transient response.
Adjustable Peak Current Control
At the beginning of a switching period, PFET Q1 is turned on and inductor current increases. Once peak current
is detected, Q1 is turned off, the diode D1 forward biases, and inductor current decreases. Figure 22 shows how
peak current detection is accomplished using the differential voltage signal created as current flows through the
current setting resistor (R_SNS). The voltage across R_SNS (V_SNS) is compared to the adjustable current sense
threshold (V_CST) and Q1 is turned off when V_SNS exceeds V_CST, providing that t_ON is greater than the minimum
possible t_ON (typically 115ns).
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