Manualshelf

Datasheet

ManualsBrandsSTMICROELECTRONICS ManualsPMICsPMIC - LED driver DC-DC voltage regulator VFQFPN 8 Surface-mount
21222324252627282930
DocID18279 Rev 5 21/37
ST1CC40 Application information
37
The LED ripple current can be calculated as the inductor ripple current ratio flowing into the
output impedance using the Laplace transform (see Figure 11):
Equation 19
where the term 8/
2
represents the main harmonic of the inductor current ripple (which has
a triangular shape) and
I
L
is the inductor current ripple.
Equation 20
so L value can be calculated as:
Equation 21
where T
OFF
is the off-time of the embedded high switch, given by 1-D.
As a consequence, the lower the inductor value (so the higher the current ripple), the higher
the C
OUT
value would be to meet the specifications.
A general rule to dimension L value is:
Equation 22
Finally the required output capacitor value can be calculated equalizing the LED current
ripple specification with the module of the Fourier transformer (see Equation 19) calculated
at F
SW
frequency.
Equation 23
Example (see Section : Example):
V
IN
= 12 V, I
LED
= 700 mA,
ILED
/I
LED
= 2%, V
FW_LED
= 3.5 V, n
LED
= 2
The output capacitor value must be dimensioned according to Equation 23.
Finally, given the selected inductor value, a 2.2 µF ceramic capacitor value keeps the LED
current ripple ratio lower than 2% of the nominal current. An output ceramic capacitor type
(negligible ESR) is suggested to minimize the ripple contribution given a fixed capacitor
value.
L
n
LED
V
FW_LED
100mV+
I
L
------------------------------------------------------------------ T
OFF
n
LED
V
FW_LED
100mV+
I
L
------------------------------------------------------------------ 1
n
LED
V
FW_LED
100mV+
V
IN
------------------------------------------------------------------–


==