■ Operation of Step-up DC/DC Converter and Output Current
Step-up DC/DC Converter makes higher output voltage than input voltage by releasing the energy accumulated
during on time of Lx Transistor on input voltage.
<Basic Circuit>
i2
Inductor
Diode
IOUT
VOUT
VIN
CL
i1
Lx Tr
GND
<Current through L>
Discontinuous Mode
Continuous Mode
ILxmax
IL
IL
ILxmax
ILxmin
ILxmin
Tf
Iconst
t
t
Ton
Toff
Ton
T=1/fosc
Toff
T=1/fosc
Step 1. Lx Tr. is on, then the current IL=i1 flows, and the energy is charged in L. In proportion to the on time of Lx Tr.
(Ton), IL=i1 increases from IL=ILxmin=0 and reaches ILxmax.
Step 2. When the Lx Tr. is off, L turns on Schottky Diode (SD), and IL=i2 flows to maintain IL=ILxmax.
Step 3. IL=i2 gradually decreases, and after Tf passes, IL=ILxmin=0 is true, then SD turns off. Note that in the case of
the continuous mode, before IL=ILxmin=0 is true, Toff passes, and the next cycle starts, then Lx Tr. turns on again.
In this case, ILxmin>0, therefore IL=ILxmin>0 is another starting point and ILx max increases.
With the PWM controller, switching times during the time unit are fixed. By controlling Ton, output voltage is
maintained.
■ Output Current and Selection of External Components
Output Current of Step-up Circuit and External Components
There are two modes, or discontinuous mode and continuous mode for the PWM step-up switching regulator
depending on the continuous characteristic of inductor current.
During on time of the transistor, when the voltage added on to the inductor is described as VIN, the current is VIN ×t/L.
Therefore, the electric power, PON, which is supplied with input side, can be described as in next formula.
ON
T
2
PON=∫VIN ×t/L dt
Formula 1
0
With the step-up circuit, electric power is supplied from power source also during off time. In this case, input current is
described as (VOUT-VIN)×t/L, therefore electric power, POFF is described as in next formula.
Rev.1.10
- 8 -
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
