AN2067 查看數據表（PDF） - STMicroelectronics

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AN2067

VIPower™: dimmable white LED power supply with the VIPer53-E

STMicroelectronics 

White LED power supply

AN2067

2.1.3

2.1.4

Secondary section

On the right side of the schematic, the secondary winding is used in flyback mode. D101,

C112 are respectively the rectifying diode and its filtering. L101 and C110 build another low-

pass filter.

The auxiliary secondary winding, used in forward mode (refer to Section 3) in association

with R122, D122 and C121, builds the rectifier and filtering for the supply of the regulation

loop, using a dedicated component (TSM101). This supply is independent from the load

voltage.

The TSM101 has been designed as voltage and current controller, which can be used for

the control of a current generator, in association with a voltage limitation. It includes its own

reference voltage (bandgap), and two operational amplifiers.

Current regulation loop

The output current is sensed through the shunt resistor R100. The shunt voltage is amplified

using R101 and R107 in association with one op-amp of the TSM101, building the error

amplifier. The current target is set through the trimmer P101. R103 and P101 provide a

fraction of the reference voltage provided by the TSM101 (U103).

There is also another way to set the current target, using the connector J107 for dimming

(see Section 5.4).

C113 with R107 is the integrator network of this amplifier, in order to cancel the static error

of the regulation loop.

The regulation loop continues with the optocoupler U2 (diode and transistor). The level of

the COMP pin filter sets the peak drain current of the VIPer53-E (current control mode).

Thus, the energy stored inside the transformer during each cycle is transferred to the

secondary side, supplying the output current.

R131 and C131 are a phase lead network to compensate the phase delay due to

L101/C110 filter.

2.2

2.2.1

2.2.2

Dimming

Dimming purpose

The main purpose of this application is to supply “single-chip white LEDs”, and to dim the

brightness of these LEDs.

Since white color is obtained by two peaks in the spectrum (a blue ray and a yellow ray),

there is a dependency on the driving current and the white color spectrum.

Dimming in the application

This application proposes how to apply an external PWM signal to the node VREF_INPUT.

Provided that output impedance of the generator is not higher than 50 Ω, the input voltage is

forced by the external generator instead of the DC reference voltage of the TSM101.

The low level voltage of this PWM signal must be 0 V, and the high level voltage must be

around 1 V. The peak current of the PWM generator can be set using the trimmer P101, as

in DC mode, from 0 up to 1 A.

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DocID10973 Rev 2

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