AN4300 datasheet(7/40 Pages) STMICROELECTRONICS

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AN4300

Lighting application design challenge

1.2

Driver signals characteristics

The signals that drive the light loads must satisfy the following requirements:

•

Capability to drive up to 60 channels with PWM signals

•

Capability to drive different load types using PWM signals with different characteristics:

–

Bulb lamp

PWM frequency: ~100 Hz

Duty cycle: 50 to 100 %

Duty cycle adjustment precision: usually 5 %

–LED

PWM frequency: 150 to 200 Hz

Duty cycle: 10 % to 100 % (as much as 2 % to 100 % for high efficiency LED)

Duty cycle adjustment precision: usually 1–2 %

The variation of the duty cycle is performed to maintain constant power on the lamps when

the battery voltage changes.

The duty cycle variation is based on the following expression:

Equation 1

where

Vref is the nominal value of the battery voltage

Vbat is the actual battery voltage

If Vbat ≤ Vref, the duty cycle is equal to 100 %, else the expression is applied. The adaptation

of the duty cycle is performed one time per period.

A typical battery voltage value for automotive applications is 13.2 V, so if, for example, the

application reads a Vbat equal to 16 V, a duty cycle of 68 % applies.

When turning the lamp light bulb on, the bulb current applied to a cold filament can reach up

to 10 times the rated current. The total current of several lamps causes a significant power

supply voltage drop and an increase in the electromagnetic interference (or EMI) emissions.

Therefore, it is essential not to drive all lamps together and it is necessary to introduce a

delay between each pair of rising edges of the PWM signals (see Figure 3).

1.3

System proposal

In a typical system solution for a light application, the SPC56xB/Cx microcontroller family

can manage several power drivers, depending on how the power is managed and the

number of channels to be driven.

Some devices are driven by an SPI link. Using this link, the application can configure the

signals, that are generated inside the driver device, to drive the loads.

bat

re f

If V b a t > V r e f

bat

re f

If V b a t > V r e f

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