An374 – Cirrus Logic AN374 User Manual
Page 9
AN374
AN374REV2
9
Constraint 3: Ratio of peak currents between the two channels must be less than 4
The DAC reference on the current sense comparator has a 1.4V full-scale threshold. There is a minimum peak
current I
PK(FB)
at full brightness threshold that is usually set at approximately 25%, so the theoretical maximum
ratio of the peak currents is 5.6. However, a range is required to dim down to the lower dimming settings.
Hence a ratio of 1.5 is considered optimum. Any peak ratio beyond 2.5 is considered a difficult requirement for
the driver to guarantee regulation across the dimming range. If the resonant reset time T3 is short and can be
neglected, then the ratio of the peak currents can be given as follows:
Refer to application note AN368 for detailed calculation.
Constraint 4: Both strings cannot have identical configuration
Either the output voltage differential should be maintained, or the output current differential should be
maintained. If the strings are placed in series, then the currents should never be the same at any point of the
dimming curve. If the strings are placed in parallel, then the voltage cannot be the same. The currents can
cross in a parallel load configuration. In a commercial lighting space that requires mixing white LEDs of
different wavelengths to produce a higher CCT white light (>3000K), it is recommended to have a different
number of white LEDs in the two LED strings when using the CS1630/31.
Constraint 5: Maximum differential between string voltages
The overvoltage protection (OVP) threshold is set to 1.25V and the FBSENSE comparator threshold is set at
a 200mV. As the dimmer conduction angle is reduced, the flyback operates continuously in DCM, while
maintaining valley switching for low losses. At very low conduction angles, the flyback may switch on the third
valley. Therefore, a minimum signal of 400mV on the FBAUX pin is recommended, which means that the string
voltage of one channel cannot be greater than three times the string voltage of the other channel in a parallel
load configuration. The FBAUX pin is used exclusively for overvoltage protection. In the event that a higher
string voltage differential is desired, an external circuit must be designed such that different divider ratios are
chosen for both modes of operation.
The OVP point is fixed for the higher of the two strings. For example, if one string has an output voltage of 5V
and the other string has an output voltage of 15V, the OVP point is still considered to be at greater than 15V.
Therefore, both output capacitors must be rated for the OVP voltage, which is in this case greater than 15V.
Constraint 6: The implications when determining LED string configuration and topology
Identify the light engine and select an appropriate second stage topology. There are two aspects that need to
be considered: first, whether to use the series or parallel LED configuration and second, which power train
topology to use.
I
PK1 FB
I
PK2 FB
-------------------
P
MODE1
P
MODE2
--------------------
=
[Eq. 6]