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MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
Figure 13. Basic Buck Converter Operation and its Waveforms
IQ
IL
ID
D
Q
Vin
RLOAD
VO
+
–
+
CO
L
Vin
RLOAD
+
–
+
CO
L
POWER SWITCH ON
ILOAD
Vout
ILO
VD(fwd)
RLOAD
+
–
+
CO
POWER SWITCH OFF
ILOAD
Vout
ILO
IO
IL
IQ
–ID
VD
VCo
DIL
ton toff
T
Vfwd
t
Boost Mode
The operation of the boost converter also consists of two
parts, when the power switch is on and off. When the power
switch turns on, the input voltage source is placed directly
across the inductor, and the current ramps up linearly
through the inductor as described by:
iL(on) +
Vin
ton
L
Where:
ton is the on–time of the power switch.
Vin is the input voltage.
iL(on) is the inductor current during the on–time.
L is the inductance of the inductor L.
The current ramping across the inductor stores energy
within the core material. In order to maintain steady–state
operation, the amount of energy stored during each switching
cycle, times the frequency of operation must be higher (to
cover the losses) than the power demands of the load:
Psto +
1
2
LI2pk
f
u Pout
When the power switch turns off again, the inductor voltage
flies back above the input voltage and is clamped by the
forward biased rectifier at the output voltage.
The current ramps down through the inductor to the output
until the new on time begins or, in case of discontinuous
mode of operation, until the energy stored in the inductor core
drops to zero.
iL(off) +
(Vo * Vin)
toff
L
Where:
toff is the off–time of the power switch.
Vo is the output voltage.
During the steady state operation iL(on) = iL(off) = IL, and
d
+
Vo * Vin
Vo
Where:
d is the duty cycle, and d = ton/T.
T is switching period, T = 1/f.
f is the frequency of operation.
The ripple voltage of the boost converter can be described
as:
VppCo +
Io
Co
(Vo * Vin)
Vo
f
Where:
VppCo is the ripple caused by output current.
The portion of the output ripple voltage caused by the ESR
of the output capacitor is:
VppESR + (Io
Vo
Vin
) 1
2
DIL)
RESR