S-8353A20MC-IQFT2X Datasheet, PDF (18/49 Pages) Seiko Instruments Inc – BUILT-IN TRANSISTOR SWITCHING REGULATOR

STEP-UP, PWM CONTROL or PWM / PFM SWITCHABLE BUILT-IN TRANSISTOR SWITCHING REGULATOR
S-8353/8354 Series
Rev.3.0_01
4. Operation
The following are the basic equations [(1) through (7)] of the step-up switching regulator. (Refer to Figure 11.)
L
CONT
D
VIN
VOUT
M1
EXT
VSS
+
− ^CL
Figure 11 Step-Up Switching Regulator Circuit for Basic Equation
Voltage at CONT pin at the moment M1 is turned ON (VA) ^*1 :
VA = VS ^*2
(1)
*1. Current flowing through L (IL) is zero.
*2. Non-saturated voltage of M1.
The change in IL over time :
dIL
dt
=
VL
L
=
VIN − VS
L
(2)
Integration of equation (2) (IL) :
I_L
=


V_IN − V_S
L
 • t

(3)
IL flows while M1 is ON (tON). The time of tON is determined by the oscillation frequency of OSC.
The peak current (IPK) after tON :
I_PK
=


V_IN −
L
V_S
 • t_ON

(4)
The energy stored in L is represented by 1/2 • L (IPK)².
When M1 is turned OFF (tOFF), the energy stored in L is emitted through a diode to the output capacitor.
Then, the reverse voltage (VL) is generated :
VL = (VOUT + VD^*1) − VIN
(5)
*1. Diode forward voltage
The voltage at CONT pin rises only by VOUT+VD.
The change in the current (IL) flowing through the diode into VOUT during tOFF :
dI_L
dt
=
V_L
L
=
V_OUT
+ V_D − V_IN
L
(6)
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