S-882Z Datasheet, PDF (10/24 Pages) Seiko Instruments Inc – ULTRA-LOW VOLTAGE OPERATION CHARGE PUMP IC FOR STEP-UP DC-DC CONVERTER STARTUP

ULTRA-LOW VOLTAGE OPERATION CHARGE PUMP IC FOR STEP-UP DC-DC CONVERTER STARTUP
S-882Z Series
Rev.1.2_00
„ Selection of Product and Startup Capacitor (C_CPOUT)
To reliably start up the step-up DC-DC converter, in the S-882Z Series, select the discharge start voltage value
(V_CPOUT1) and the capacitance value of the external startup capacitor (C_CPOUT) according to the step-up DC-DC
converter to be started, its external parts, and the output load.
Generally, an output smoothing capacitor with a large capacitance value and an output load are connected to the
output pin of the step-up DC-DC converter. Therefore, to start up the step-up DC-DC converter thus connected, it
is necessary to select a large capacitance value for the startup capacitor.
As a reference, the following shows an example that a Schottky diode (SD1) is added between the output pin
(OUT) and the power supply pin (VDD) of the step-up DC-DC converter to be started up in the S-882Z Series
(refer to Figure 14). As a result, it is possible to start up the step-up DC-DC converter by the capacitor with a
small value.
C_VDD in Figure 14 is the power supply smoothing capacitor of the step-up DC-DC converter.
VIN
CCPOUT
CPOUT
VIN
S-882Z Series
OUT
VM
CIN
VSS
SD1
CVDD
VDD
VIN
OUT
Step-up DC-DC converter
VSS
VOUT
CL
Figure 14 Connection Diagram (Reference)
The selection method for V_CPOUT1 and C_CPOUT when the reference connection shown in Figure 14 has been
performed is described below.
Select V_CPOUT1 and C_CPOUT so that they satisfy the following conditional expression.
V_CPOUT1 • C_CPOUT
C_CPOUT + C_VDD
− 0.1 • I_VDD − V_DDL
V_CPOUT1 > V_DDL + 0.2 V
• (C_CPOUT + C_VDD) > 2t_S • I_VDD
C_CPOUT > 10C_VDD
V_CPOUT1 : Discharge start voltage value of S-882Z Series (unit : V)
C_CPOUT : Capacitance value of startup capacitor (unit : µF)
C_VDD : Capacitance value of power smoothing capacitor for step-up DC-DC converter (unit : µF)
I_VDD : Consumption current value of step-up DC-DC converter (unit : mA)
V_DDL : Minimum operation voltage of step-up DC-DC converter (unit : V)
t_S :
Step-up DC-DC converter startup time ≅ Soft start time (unit : ms)
10
Seiko Instruments Inc.