34704_09 Datasheet, PDF (37/49 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
• Now calculate the maximum allowed ESR to reach the
desired ΔVO_r.
ESR ≤ ----------------^Δ----V----o---^r---------------
⎛
⎝
----I---o---^m----^a---^x----
1 – D_max
+
I_{OB⎠⎞}
[Ω]
• 1CVG: Use a 47uF capacitor from Ground to VG.
• D1: Use a fast recovery schottky diode rated to 10V at 1A.
Regulator 2, 4 and 5 (Synchronous Buck-Boost regulator
with external compensation)
These three regulators are 4-Switch synchronous buck-boost
voltage mode control DC-DC regulator that can operate at
various output voltage levels. Since each of the regulators may
work as a buck or a boost depending on the operating voltages,
they need to be compensated in different ways for each
situation.
Since the 34704 is meant to work using a LiIon battery, the
operating input voltage range is set from 2.7 - 4.2 V, then the
following scenarios are possible:
Regulator
Vo
Input voltage
range
Operation
2
2.8 V
3.0 - 4.2
Buck
3.3 V
2.7 - 3.0
Boost
3.3 V
3.5 - 4.2
Buck
4
1.8 V
2.7 - 4.2
Buck
2.5 V
2.7 - 4.2
Buck
5
3.3 V
2.7 - 3.0
Boost
3.3 V
3.5 - 4.2
Buck
• NOTE: Since these 3 regulators can work as a buck or a
boost in a single application, a good practice to configure
these regulators is to compensate for a boost scenario and
then verify that the regulator is working in buck mode using
that same compensation.
Compensating for Buck operation:
• 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.
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
≈
D′_M
A
X
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.
1. 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---_{ms---w-a--x-⎠⎞}
⎛
⎝
-V----i--n---^m----ⁱ_L--ⁿ----–----V-----o--_⎠⎞
[A]
2. Then the output capacitor can be chosen as follow:
C_OUT ≤ ^I-_Δ--o--V-^m----o^a--^x-_m--d--_a--t-_x-I
[A]
• Where ΔVO_max is the maximum allowed transient
overshoot expressed as a percentage of the output
voltage, typically from 3 to 5% of Vo.
3. Finally find the maximum allowed ESR to allow the
desired transient response:
ESR_max = ^Δ--V---V--o--o-₍--^r-1-⁽---F–---s--D-w---_m--⁾--⁽_i--_n-L--₎--⁾
[Ω]
NOTE: Do not use the parameters ΔVO_r and ΔVO_max
indistinctly, the first one indicates the output voltage ripple, while
the second one is the maximum output voltage overshoot
(transient response).
• R1 and RB: These two resistors help to set the output voltage
to the desire value using a Vref=0.6V, select R1 between 10k
and 100K and then calculate RB as follows:
RB = -------R-----1--------
---V----o---- – 1
[Ω]
Vref
• Compensation network. (C1,C2,C3, R2, R3): For
compensating a buck converter, 3 important frequencies
referring to the plant are:
Analog Integrated Circuit Device Data
Freescale Semiconductor
34704
37