R1283x
10
There are also two operation modes for the PWM control inverting switching regulator, that is the continuous
mode and the discontinuous mode.
When the LX Tr. is on, the voltage for the inductor L will be VIN. The inductor current (IL1) will be;
IL1 = VIN×Ton / L..................................................................................................................Formula8
Inverting circuit saves energy during on time of Lx Tr,and supplies the energy to output during off time,output
voltage opposed to input voltage is obtained. The inductor current at off (IL2) will be;
IL2 = VOUT×Tf / L .................................................................................................................Formula9
(The above formula and after, the absolute value of the negative output voltage is assumed to be
VOUT.
:Output voltage= -10V ,VOUT=10 )
In terms of the PWM control, when the Tf=Toff, the inductor current will be continuous, the operation of the
switching regulator will be continuous mode.
In the continuous mode, the current variation of IL1 and IL2 are same, therefore
VIN×Ton / L = VOUT×Toff / L .................................................................................................Formula10
In the continuous mode, the duty cycle will be:
DUTY = Ton / (Ton+Toff) = VOUT / (VOUT + VIN )......................................................................Formula11
If the input power equals to output power,
IOUT = VIN
2×Ton / (2×L×V
OUT) ............................................................................................Formula12
When IOUT becomes more then Formula12 ,it will be continuous mode.
In this moment ,the peak current,Ilxmax flowing through the inductor is described as follows:
ILxmax = IOUT×VOUT / VIN + VIN×Ton / (2×L)........................................................................Formula13
ILxmax = IOUT×VOUT / VIN + VIN×VOUT×T / { 2×L×(VOUT + VIN ) }........................................Formula14
Therefore,peak current is more than IOUT.Considering the value of Ilxmax,the condition of input and
output,and external components should be selected.
The explanation above is based on the ideal calculation,and the loss caused by Lx switch and external
components is not included.
The actual maximum output current is between 50% and 80% of the calculation.
Especially,when the IL is large,or VIN is low,the loss of VIN is generated with on resistance of the switch.As
for VOUT,VF(as much as 0.3V)of the diode should be considered.
Discontinuous Mode
ILxmax
ILxmin
Ton
Toff
T=1/fosc
Tf
IL
t
ILxmax
ILxmin
Ton
Toff
T=1/fosc
t
IL
Continuous Mode
<
Current through L>