34704_09 Datasheet, PDF (41/49 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
string. It does not need external compensation network, thus
the only components that need to be calculated are:
• 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:
• Define I_OB as the minimum current to maintain CCM as
15% of full load:
L_m
i
n
≥
V-----o----(_--D-----)_--(_{--1-----–-----D----})_--²_---T--
2I_OB
[H]
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 ÄI_L, then:
L_min
≥
V-----o----(_--T----)
16I_OB
[H]
• C_OUT: 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:
C_OUT ≥ ^I--F-o---m_s---w-^a--^x-_Δ--D--V---^m--o--^a-_r-^x-
[F]
• Where ΔVO_r is the desired output voltage ripple.
• Now calculate ΔVO_r the maximum allowed ESR to
reach the desired.
ESR ≤ ----------------^Δ---V-----o---^r---------------
[Ω]
⎛
⎝
----I---o---^m----^a--^x-----
1 – D_max
+
I_{OB⎠⎞}
• R1 and RB (for Voltage feedback control): These two
resistors help to set the output voltage to the desire value
using a V_REF=0.6V, select R1 between 10k and 100K and
then calculate RB as follows:
RB = -------R-----1--------
[Ω]
---V----o---- – 1
Vref
• RS (For current feedback control with LED string):
This resistor is attached at the end of the LED string and it
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:
•
RS = -V----^r--^e--^f
[Ω]
Io
Analog Integrated Circuit Device Data
Freescale Semiconductor
Where Vref=230mV is the maximum internal reference
voltage in current mode control that is reflected on the FB8
pin.
Regulator 7 (Inverter controller - external compensation
needed)
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 -7V at a maximum
current of 60 mA.
• P-MOSFET: The peak current of the MOSFET is assumed
to be I_D, which is obtained by the following formula, define
I_OB from 5 to 15% of maximum current rating.
I_Q ≥ I_Lpeak
=
–----(_{--I---o----+-----I--O----B----})
1–D
And the voltage rating is given by:
V_Q = Vin – Vo
• Diode D7: The peak value of the diode current is I_FSM
which should also be higher than I_Lpeak. The average
current rating should be higher than the output current low
and the repetition reverse voltage V_RRM is given by:
V_RRM ≥ Vin – Vo
• L: The minimum value of inductor to maintain CCM can be
determined by using the following procedure:
L_min
≥
--–---V-----o---T----
2Io_max
⎛
⎝
V-----o--V---–--i--nV---^m--i--ni--n-_m^----_i^--_n^{-⎠⎞}
2
[H]
• C_OUT: 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:
C_OUT ≥ -I-F-o---_Sm---_W-^a---^x-_Δ-D--V---^m--o--^a-_r-^x-
[F]
• Where ΔVO_r is the desired output voltage ripple.
• Now calculate the maximum allowed ESR to reach the
desired.
ESR ≤ ------------------^Δ----V----o---^r------------------
[Ω]
⎛
⎝
----I---o---^m----^a--^x-----
1 – D_max
+
1---I--^O–----^{B-D--⎠⎞}
• R1 and RB: These two resistors help to set the output
voltage to the desire value using a VFB7=0.6V, select R1
between 10k and 150K and then calculate RB as follows:
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