LED5000 datasheet(18/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

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Doc ID 023951 Rev 1

With the power components selected in accordance with

Chapter 5.9: Component selection

and given the BW specification, the components composing the compensation network can

be calculated as:

Equation 16

where the term mC is represented in Equation 4, RLOAD the equivalent loading resistor

(

Equation 12), RS the sensing resistor value, Gm the error amplifier transconductance and

RCS the current sense gain (Table 5: Electrical characteristics)

Equation 17

where K represents the leading position of the FZ (Equation 11) with respect to the system

bandwidth. In general, a value of 2 gives enough phase margin to the overall small loop

transfer function.

5.7

Example of system design

Design specification:

VIN=48 V, VFW_LED=3.7 V, nLED= 10, rLED= 1.1 W, ILED= 1 A, ILED RIPPLE= 2%

The inductor and capacitor value are dimensioned to meet the ILED RIPPLE specification (see

Chapter 5.9.2 for output capacitor and inductor selection guidelines):

L=22

μH, COUT=1.0 μF mlcc (negligible ESR)

In accordance with

Chapter 5.9.1 the sensing resistor value is:

Equation 18

Assuming a system bandwidth of:

Equation 19

the ideal values of the components making up the compensation network is:

Equation 20

Final component selection is based on commercial values and a small capacitor CP is

added to reduce noise at the error amplifier output. CP slightly decreases the BW and phase

margin.

LOAD

⋅

------------------------------------

–

() 0.5

–

⋅

[]

⋅

-----------------------------------------------------------------------------------------------------------

⋅

----------------------------

⋅

------------------------

200 mV

1 A

--------------------

200 m

70 kHz

MAX

43 k

650 pF

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