LM3409 Datasheet, PDF (9/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

FIGURE 1. Ideal CCM Buck Converter Inductor Current i_L(t)
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CONTROLLED OFF-TIME (COFT) ARCHITECTURE
The COFT architecture is used by the LM3409/09HV to con-
trol I_LED. It is a combination of peak current detection and a
one-shot off-timer that varies with output voltage. D is indi-
rectly 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 com-
pensation necessary in many switching regulators, simplify-
ing 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 detect-
ed, Q1 is turned off, the diode D1 forward biases, and inductor
current decreases. Figure 2 shows how peak current detec-
tion is accomplished using the differential voltage signal cre-
ated 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).
FIGURE 2. Peak Current Control Circuit
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