34704_11 Datasheet, PDF (40/54 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
1. Define I_OB as the minimum current to maintain CCM as
15% of full load.
L_min
≥
V_-----o_----(_--D_-----)_--(_---1_----–_-----D_-----)_--²_---T_--
2I_OB
(H)
where: D = Dutycycle
Vo = Output Voltage
T = Switching Period
IOB = Boundary Current to achieve CCM
2. 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 ΔI_L,
then:
L_min
≥
V_-----o_----(_--T_----)
16 I _{O B}
(H)
Note: On the 34704B Use the recommended 3.0uH
inductor rated between 50 to 100 mA in order to have this
regulator working in DCM. Rising the inductor value will make
the regulator to begin working in CCM.
• 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_---o_{---m----a---x---}D_{-----m----a---x}
FswΔVo_r
(F)
where: Dmax = Maximum Dutycycle
FSW = Switching Frequency
• Where ΔVO_r is the desired output voltage ripple.
• Now calculate the maximum allowed ESR to reach the
desired ΔVO_r.
E
S
R
≤
----------------^Δ----^V-----^o---^r----------------
⎛
⎝
-----^I--^o---^m-----^a---^x-----
1 – D_max
+
I_O
⎞
B_⎠
[Ω]
• 1CVG (Only Reg1): Use a 47uF capacitor from Ground to
VG.
• D1 (Only Reg1): 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
2
4
5
Vo
2.8 V
3.3 V
3.3 V
1.8 V
2.5 V
3.3 V
3.3 V
Input voltage
range
Operation
3.0 - 4.2
Buck
2.7 - 3.0
Boost
3.5 - 4.2
Buck
2.7 - 4.2
Buck
2.7 - 4.2
Buck
2.7 - 3.0
Boost
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.
Lm
i
n
≥
-⁽--^V----^o------⁺-----^I--^o---^m------^a----^x---⁽---^R----^D-----^S----^O-----^N-----^L----^S----^F-----^E----^T-----⁺-----^R-----^L----⁾--^D-----^′--^m-----ⁱ--ⁿ----⁾--^T---
2IOB
≈
D
′_M
A
X
T
----^V----^o------
2IOB
[H]
where: RDSONLSFET = Body Resistance of the Lowside Fet
RL = Inductor Winding Resistance
D'Min = Minimum Off Percentage given by 1- (Vin_min/Vout_max)
D'max = Maximum Off Percentage given by 1- (Vin_max/Vout_min)
• 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.
34704
40
Analog Integrated Circuit Device Data
Freescale Semiconductor