34704_09 Datasheet, PDF (40/49 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
The passive components associated to these frequencies are calculated with the following formulas
C1
=
-V-----i--n---^m----ⁱ--ⁿ-
V_RAMP
⎛
⎜
⎝
D-----′^--_m^1----_i^--_n^{--2-⎠⎟⎞}
⎛
⎝
-2---_π---₍---F---_P-1--_O----R-----1---_{)⎠⎞}
C2
=
⎛
⎝
2----π^---(^---F---1_Z^----₂^--R-----1---)^{-⎠⎞}
R2
=
⎛
⎝
2----π^---(^---F---1_Z^----₁^--C-----1---)^{-⎠⎞}
R3
=
⎛
⎝
2----π^---(^---F---1_P^---₁^{---C----2----})^⎠⎞
C3
=
⎛
⎝
2----π^---(^---F---1_P^---₂^{---R----2----})^⎠⎞
On the 34704 V_RAMP is half of 1.2V since each operation mode spends only half the ramp.
Regulator 3 (Synchronous Buck - internally
compensated)
• L: A buck 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:
1. Define I_OB as the minimum current to maintain CCM as
15% of full load.
C_OUT
≤
I---o---_{m----a--x---d----t--I}
ΔV o _{m a x}
[F]
Where ΔVO_max is the maximum allowed transient
overshoot expressed as a percentage of the output voltage,
typically from 3 to 5% of Vo.
• Finally find the maximum allowed ESR to allow the
desired transient response:
L_min
≥
-⁽--V-----o----+-----I--o----^m----^a--^x---⁽---R----^D---^S---^O----^N----^L---^S---^F---^E---^T-----+-----R----^L---⁾--⁽---D----^′--^m----ⁱ--ⁿ---⁾--T--
2I_OB
L_min
≥
D ′ T ---V-----o---
2I_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. A good
approach to calculate the minimum real capacitance
needed is to include the transient response analysis to
control the maximum overshoot as desired.
• First calculate the dt_I (inductor current rising time)
given by:
dtI = -I---o---^m----^a--^x---T---
[s]
ΔIostep
Where the parameter ΔIO_{_}step is the maximum current
step during the current rising time and is define as:
ΔIostep
=
⎛
⎝
-D-F---^m_s---w-^a--^x-_⎠⎞
⎛
⎝
-V----i--n---^m----ⁱ_L--ⁿ----–----V-----o--_⎠⎞
[A]
• Then the output capacitor can be chosen as follow:
ESR_max
=
-^Δ---V-----o---^r--⁽--F----s---w-----⁾--⁽--L----⁾-
Vo(1 – D_min)
[Ω]
NOTE: do not use the parameters ΔVO_R and ΔVO_max
indistinctly, the first one indicates the output voltage rip-
ple, while the second one is the maximum output volt-
age overshoot (transient response).
• R1 and RB: 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
Regulator 8 (Synchronous Boost - internally
compensated -Voltage or current feedback)
REG8 is a Synchronous Boost converter set to 15V with a
maximum current of 30mA and can be used with voltage
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
34704
40
Analog Integrated Circuit Device Data
Freescale Semiconductor