AN4460 datasheet(19/54 Pages) STMICROELECTRONICS

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AN4460

SMED implementation

4.3

Input/output signals

The SMED uses the following list of input/output signals and pins:



PWMx: PWM output signal

The signal is fed into the MOSFET's driver input to control the MOSFET's

switching state. The PWM operates according to the TON and TOFF time faced in

Section 2 on page 8. When the PWM is high, the MOSFET is ON and vice versa.

When the supply voltage of the STLUX385A is 5 V, the output signal can be

directly connected to the MOSFET. Instead, when the supply voltage is 3.3 V, the

PM8834 buffer is required.



CPPx: comparator input signal

CPPx receives the current sense signal and compares it to the internal DAC value.

When the voltage on CPPx pin is higher than the DAC value, the output of the

internal analog comparator goes high, and vice versa. The comparator output is

the event(1) internal signal. Event(1) is programmed at level mode so that the

SMED can detect immediately the eventual anomaly during S(1). If edge mode

was used and the anomaly happened outside S(1) then S(1) wouldn't receive the

edge event. There are only two SMED states that check for this event: fault zone

(S1) and current limit (S2).



SWx: software event

Used to start and pause the SMED. When the SMED is paused, the PWM output

has to be set to 0. The SWx event is used during S(0) to pause the SMED and

during HOLD to restart. The event is programmed at level mode and is used by

the software to define the dimming duty cycle period. Thanks to the event mode,

the SW trigger is detected by S(0) even if the SW edge happens while the SMED

is in a different state then S(0). In this case, the SMED completes the cycle,

reaches S(0), captures the SW level and goes on HOLD.



DIGINx: zero current detection

The pin is used for zero current detection while it is in transition mode. At the

moment the transition mode is a proposal and the pin is not managed.



VCOM0

The voltage applied to the pin is read by the STLUX385A ADC and it is used to

compute the TON time and the TOFF time. The voltage is captured 100 ns after the

beginning of the ON portion of the dimming cycle as the current is expected to

reach the correct value.



VLED_PW

The voltage applied to this pin is read by the STLUX385A ADC and is used to

compute the TON time and the TOFF time. It is acquired at the same time of the

VCOM0 voltage using the 8 values, four for VCOM0 and four for VLED_PW -

interleaved, stored by the ADC in registers ADC_DATH< 7-0 > and ADC_DATL<

7-0>.

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