S-8351A50MC-J3JT2G Datasheet, PDF (18/51 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
S-8351/8352 Series
Rev.3.0_00
3. Operation
The following are the basic equations [(1) through (7)] of the step-up switching regulator. (Refer to Figure 18.)
L
VIN
ON / OFF
Di
CONT
VOUT
+
− ^CL
OSC
M1
+
−
Figure 18 Step-Up Switching Regulator Circuit for Basic Equation
Voltage at CONT pin at the moment M1 is turned ON (V_A) ^*1 :
V_A = V_S ^*2
(1)
*1. Current flowing through L (I_L) is zero.
*2. Non-saturated voltage of M1.
The change in I_L over time :
dIL ₌ VL ₌ VIN − VS
(2)
dt L
L
Integration of equation (2) (I_L) :
I_L
=
⎜⎛
⎝
V_IN − V_S
L
⎟⎞ • t
⎠
(3)
I_L flows while M1 is ON (t_ON). The time of t_ON is determined by the oscillation frequency of OSC.
The peak current (I_PK) after t_ON :
I_PK
=
⎜⎛
⎝
V_IN −
L
V_S
⎟⎞
⎠
•
t_ON
(4)
The energy stored in L is represented by 1/2 • L (I_PK)².
When M1 is turned OFF (t_OFF), the energy stored in L is emitted through a diode to the output capacitor.
Then, the reverse voltage (V_L) is generated :
V_L = (V_OUT + V_D^*1) − V_IN
(5)
*1. Diode forward voltage
The voltage at CONT pin rises only by V_OUT+V_D.
The change in the current (I_L) flowing through the diode into V_OUT during t_OFF :
dI_{L =} V_{L =} V_OUT + V_D − V_IN
(6)
dt L
L
18
Seiko Instruments Inc.