FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
feedback using the standard voltage divider configuration, or
can be programmed to work with a current feedback
configuration to control the current flowing through a LED
string. It does not need external compensation network, thus
the only components that need to be calculated are:
controls the amount of current flowing through it. To
calculate this resistor, set the maximum current you want
to flow though the string and use the following formula:
• L: A boost power stage can be designed to operate in
CCM for load currents above a certain level usually 5 to
15% of full load. The minimum value of inductor to
maintain CCM can be determined by using the following
procedure:
Vref
---------
Io
[Ω]
RS =
Where VREF=230 mV is the maximum internal reference
voltage in current mode control that is reflected on the FB8
pin.
When Input voltage is equal to or higher than VOUT8, a
reverse bias diode is needed from the switching node to the
output in order to cause a drop from the Input to the output,
see Figure 9 below:
• Define IOB as the minimum current to maintain CCM as
15% of full load:
Vo(D)(1 – D)2T
[H]
------------------------------------------
≥
Lmin
2IOB
VIN
BT8
CBOOT
However the worst case condition for the boost power
stage is when the input voltage is equal to one half of the
output voltage, which results in the Maximum ÄIL, then:
L8
SW8
D8
VOUT8
VOUT8
Vo(T)
16IOB
[H]
---------------
≥
Lmin
R1
FB8
• COUT: The three elements of output capacitor that
contribute to its impedance and output voltage ripple are
the ESR, the ESL and the capacitance C. The minimum
capacitor value is approximately:
RB
Figure 9. Reverse Bias Diode
IomaxDmax
----------------------------
≥
Regulator 7 (Inverter controller - external compensation
needed)
[F]
COUT
FswΔVor
REG7 is a non-synchronous buck/boost inverting PWM
voltage-mode control DC-DC regulator that drive an external
P-MOSFET to supply a typical voltage of -7.0 V at a
maximum current of 60 mA.
• Where ΔVOr is the desired output voltage ripple.
• Now calculate ΔVOr the maximum allowed ESR to
reach the desired.
• P-MOSFET: The peak current of the MOSFET is assumed
to be ID, which is obtained by the following formula, define
IOB from 5 to 15% of maximum current rating.
ΔVor
--------------------------------------------
Iomax
[Ω]
ESR ≤
⎛
⎝
⎞
⎠
-----------------------
+ IOB
–(Io + IOB
----------------------------
=
)
1 – Dmax
IQ ≥ ILpeak
1 – D
• R1 and RB (for Voltage feedback control): These two
resistors help to set the output voltage to the desire value
using a VREF=0.6 V, select R1 between 10k and 100K and
then calculate RB as follows:
And the voltage rating is given by:
VQ = Vin – Vo
• Diode D7: The peak value of the diode current is IFSM
which should also be higher than ILpeak. The average
current rating should be higher than the output current low
and the repetition reverse voltage VRRM is given by:
[Ω]
R1
---------------------
RB =
Vo
-----------
– 1
Vref
• RS (For current feedback control with LED string):
This resistor is attached at the end of the LED string and it
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
45
FREESCALE [ Freescale ]
