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SNVS574E – JULY 2008 – REVISED MAY 2013
Design Guide
Refer to the Basic Topology Schematics section.
SPECIFICATIONS
Number of series LEDs: N
Single LED forward voltage: V LED
Single LED dynamic resistance: r LED
Nominal input voltage: V IN
Input voltage range: V IN-MAX , V IN-MIN
Switching frequency: f SW
Current sense voltage: V SNS
Average LED current: I LED
Inductor current ripple: Δ i L-PP
LED current ripple: Δ i LED-PP
Peak current limit: I LIM
Input voltage ripple: Δ v IN-PP
Output OVLO characteristics: V TURN-OFF , V HYSO
Input UVLO characteristics: V TURN-ON , V HYS
1. OPERATING POINT
Given the number of series LEDs (N), the forward voltage (V LED ) and dynamic resistance (r LED ) for a single LED,
solve for the nominal output voltage (V O ) and the nominal LED string dynamic resistance (r D ):
V O = N x V LED
r D = N x r LED
Solve for the ideal nominal duty cycle (D):
Buck
(34)
(35)
D=
Boost
D=
V O
V IN
V O - V IN
V O
(36)
(37)
Buck-boost
D=
V O
V O + V IN
(38)
Using the same equations, find the minimum duty cycle (D MIN ) using maximum input voltage (V IN-MAX ) and the
maximum duty cycle (D MAX ) using the minimum input voltage (V IN-MIN ). Also, remember that D' = 1 - D.
2. SWITCHING FREQUENCY
Set the switching frequency (f SW ) by assuming a C T value of 1 nF and solving for R T :
Copyright ? 2008–2013, Texas Instruments Incorporated
31
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