FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
1. Define IOB as the minimum current to maintain CCM as
15% of full load.
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:
Vo(D)(1 – D)2T
(H)
------------------------------------------
≥
Lmin
2IOB
where: D = Dutycycle
Vo = Output Voltage
Input voltage
Regulator
Vo
Operation
T = Switching Period
IOB = Boundary Current to achieve CCM
range
2. However the worst case condition for the boost power
stage is when the input voltage is equal to one half of
2
2.8 V
3.3 V
3.3 V
1.8 V
2.5 V
3.3 V
3.3 V
3.0 - 4.2
2.7 - 3.0
3.5 - 4.2
2.7 - 4.2
2.7 - 4.2
2.7 - 3.0
3.5 - 4.2
Buck
Boost
Buck
Buck
Buck
Boost
Buck
the output voltage, which results in the Maximum ΔIL,
then:
4
5
Vo(T)
16IOB
(H)
---------------
≥
Lmin
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.
• COUT: 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:
• 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.
IomaxDmax
(F)
----------------------------
≥
COUT
Compensating for Buck operation:
FswΔVor
• 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:
where: Dmax = Maximum Dutycycle
FSW = Switching Frequency
• Where ΔVOr is the desired output voltage ripple.
1. Define IOB as the minimum current to maintain CCM as
15% of full load.
• Now calculate the maximum allowed ESR to reach the
desired ΔVOr.
(Vo + Io
(R
+ R )D′
)T
min
max DSONLSFET
L
Vo
---------------------------------------------------------------------------------------------------------------------------
--------------
L
≥
≈ D′
T
min
MAX
2I
2I
OB
OB
ΔVor
--------------------------------------------
ESR ≤
[Ω]
[H]
Iomax
⎛
⎞
-----------------------
+ IOB
⎝
⎠
1 – Dmax
where: RDSONLSFET = Body Resistance of the Lowside Fet
RL = Inductor Winding Resistance
• 1CVG (Only Reg1): Use a 47uF capacitor from Ground to
D'Min = Minimum Off Percentage given by 1- (Vin_min/Vout_max)
D'max = Maximum Off Percentage given by 1- (Vin_max/Vout_min)
VG.
• D1 (Only Reg1): Use a fast recovery schottky diode rated
• COUT: 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.
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
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
40
FREESCALE [ Freescale ]
