34704_11 Datasheet, PDF (44/54 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 1.2 V 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.
• Then the output capacitor can be chosen as follow:
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.
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:
• Finally find the maximum allowed ESR to allow the
desired transient response:
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=0.6 V, select R1
between 10 k and 100 K and then calculate RB as follows:
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]
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 30 mA and can be used with voltage
34704
44
Analog Integrated Circuit Device Data
Freescale Semiconductor