A5974AD datasheet(40/49 Pages) STMICROELECTRONICS

40 / 49 page

Application information

A5974AD

40/49

Doc ID 018762 Rev 1

As a consequence, the maximum output current is:

Equation 46

where ISW MAX represents the rated current of the device.

Figure 28 shows a tested circuit to implement a boost current source for high current LED

driving (350 mA). To implement a boost conversion the LED string must be composed of a

minimum device number having a total voltage drop larger than maximum input voltage.

The input voltage can be either DC or AC thanks to the input bridge rectifier. In the case of a

DC voltage source D1, D2, D3, D4, C1, and C2 can be removed from the circuit and 1 μF

capacitor value can be used for C5.

8.9

Synchronization example

See Section 5.3 for details.

Figure 29.

Synchronization example

8.10

Compensation network with MLCC at the output

The A5974AD standard compensation network (please refer to Figure 1 and Section 7)

introduces a single zero and a low frequency pole in the system bandwidth, so a high ESR

output capacitor must be selected to compensate the 180-degree phase shift given by the

LC double pole.

The selection of the output capacitor must guarantee that the zero introduced by this

component is inside the designed system bandwidth and close to the frequency of the

double pole introduced by the LC filter. A general rule for the selection of this compound for

the system stability is provided in Equation 47.

Equation 47

OUT MAX

SW MAX

–

()

⋅

≅

AM09655v1

A5974AD

Z ESR

π ESR C

OUT

⋅⋅

⋅

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

bandwidth

Z ESR

10 f

⋅

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