|
S-8351A50MC-J3JT2G Datasheet, PDF (19/51 Pages) Seiko Instruments Inc – STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER | |||
|
◁ |
STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER
Rev.3.0_00
S-8351/8352 Series
Integration of the equation (6) is as follows :
IL
= IPK
â ââ
â
VOUT
+ VD
L
â VIN
ââ ⢠t
â
(7)
During tON, the energy is stored in L and is not transmitted to VOUT. When receiving the output current (IOUT) from
VOUT, the energy of the capacitor (CL) is consumed. As a result, the pin voltage of CL is reduced, and goes to the
lowest level after M1 is turned ON (tON). When M1 is turned OFF, the energy stored in L is transmitted through the
diode to CL, and the voltage of CL rises rapidly. VOUT is a time function, and therefore indicates the maximum value
(ripple voltage (VPâP) ) when the current flowing through into VOUT and load current (IOUT) match.
Next, the ripple voltage is determined as follows.
IOUT vs. t1 (time) from when M1 is turned OFF (after tON) to when VOUT reaches the maximum level :
IOUT
=
IPK
â
ââ
â
VOUT
+ VD
L
â
VIN
ââ â¢
â
t1
(8)
â´ t1
=
(IPK
â IOUT ) â¢
ââââ
VOUT
L
+ VD
â
VIN
âââ â
(9)
When M1 is turned OFF (tOFF), IL = 0 (when the energy of the inductor is completely transmitted).
ââââ
VOUT
L
+ VD
â
VIN
âââ â
=
tOFF
IPK
Based on equation (7) :
(10)
When substituting equation (10) for equation (9) :
t1
=
tOFF
â
ââââ
IOUT
IPK
âââ â
â¢
tOFF
(11)
Electric charge ÎQ1 which is charged in CL during t1 :
â« â« â« ÎQ1 =
t1
0 ILdt
=
IPK
â¢
t1
dt
â
VOUT
+
VD
â
VIN
â¢
0
L
t1
tdt
0
= IPK
⢠t1
â
VOUT
+ VD
L
â
VIN
â¢
1
2
t12
(12)
When substituting equation (12) for equation (9) :
( ) ÎQ1
=
IPK
â
1
2
IPK â IOUT
â¢
t1
=
IPK
+ IOUT
2
â¢
t1
(13)
A rise in voltage (VPâP) due to ÎQ 1 :
VPâP
=
ÎQ1
CL
=
1
CL
⢠ââ IPK
â
+ IOUT
2
ââ
â
â¢
t1
(14)
When taking into consideration IOUT to be consumed during t1 and the Equivalent Series Resistance (RESR) of CL :
VPâP
=
ÎQ1
CL
=
1
CL
⢠ââ IPK
â
+ IOUT
2
ââ ⢠t1+ ââ IPK
â â
+ IOUT
2
ââ
â
â¢
RESR
â
IOUT ⢠t1
CL
(15)
When substituting equation (11) for equation (15) :
VPâP
=
(IPK
â IOUT )2
2IPK
â¢
tOFF
CL
+ ââ IPK
â
+ IOUT
2
ââ
â
â¢
RESR
(16)
Therefore to reduce the ripple voltage, it is important that the capacitor connected to the output pin has a large
capacity and a small RESR.
Seiko Instruments Inc.
19
|
▷ |