LM3409HVMYX National Semiconductor, LM3409HVMYX Datasheet - Page 9

• Manufacturer Part Number: LM3409HVMYX
• Description: LM3409 LM3409HV PFET Buck Controller for High Power LED Drives; Package: MINI SOIC EXP PAD; No of Pins: 10; Qty per Container: 3500/Reel
• Manufacturer: National Semiconductor

CONTROLLED OFF-TIME (COFT) ARCHITECTURE
The COFT architecture is used by the LM3409/09HV to con-
trol I
one-shot off-timer that varies with output voltage. D is indi-
rectly controlled by changes in both t
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.
LED
. It is a combination of peak current detection and a
FIGURE 1. Ideal CCM Buck Converter Inductor Current i
OFF
and t
FIGURE 2. Peak Current Control Circuit
ON
, which vary
9
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
adjustable current sense threshold (V
off when V
the minimum possible t
SNS
). The voltage across R
SNS
exceeds V
ON
L
CST
(t)
(typically 115ns).
, providing that t
SNS
(V
30085604
SNS
CST
) is compared to the
30085605
) and Q1 is turned
ON
is greater than
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