S8880A Datasheet, PDF (11/30 Pages) Seiko Instruments Inc – ENERGY HARVESTING POWERED BY ULTRA-LOW POWER AND ULTRA-LOW VOLTAGE OPERATION BOOST CHARGE PUMP FOR STEP-UP DC-DC CONVERTER STARTUP

ENERGY HARVESTING POWERED BY ULTRA-LOW POWER AND ULTRA-LOW VOLTAGE OPERATION BOOST CHARGE PUMP FOR STEP-UP DC-DC CONVERTER STARTUP
Rev.1.0_01
S-8880A Series
 Selection of Startup Capacitor (C_CPOUT)
To reliably start up the step-up DC-DC converter, in the S-8880A Series, select the discharge start voltage (V_CPOUT1) and
the capacitance 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 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 for C_CPOUT.
The selection method for V_CPOUT1 and C_CPOUT in Figure 5 of the connection example is described below.
Select V_CPOUT1 and C_CPOUT so that they satisfy the following conditional expressions.
V_CPOUT1 × C_CPOUT
  CCPOUT + CVDD
− 0.1 × I_VDD − V_DDL
× (C_CPOUT + C_VDD) > 2 × ts × I_VDD ･･････････････ (1)
V_CPOUT1 > V_DDL + 0.2 V
C_CPOUT > 10 × C_VDD
V_CPOUT1:
C_CPOUT:
C_VDD:
I_VDD:
V_DDL:
t_S:
Discharge start voltage of S-8880A Series (Unit: V)
Capacitance of startup capacitor (Unit: μF)
Capacitance of power smoothing capacitor for step-up DC-DC converter (Unit: μF)
Current consumption of step-up DC-DC converter (Unit: mA)
Minimum operation voltage of step-up DC-DC converter (Unit: V)
Step-up DC-DC converter startup time ≅ Soft start time (Unit: ms)
If the Schottky diode (SD1) has a large reverse current or if a pull-down resistor is added to the OUT pin of the S-8880A
Series, add the current value generated from these to the current consumption (I_VDD) of the step-up DC-DC converter.
Moreover, if SD1 is added between the OUT pin of the S-8880A Series and the power supply pin (VDD) of the step-up
DC-DC converter, set the discharge start voltage (V_CPOUT1) so that it is higher by the amount corresponding to the forward
drop voltage (V_F) of the added SD1 in comparison to V_CPOUT1 obtained with formula (1).
Caution 1. The S-8880A Series can start up the step-up DC-DC converter more reliably as the discharge start
voltage (V_CPOUT1) is higher and the capacitance of the startup capacitor (C_CPOUT) is larger. However,
note that the time from when the power is input until the step-up DC-DC converter starts becomes
longer in this case.
2. In the S-8880A Series, the capacitance of the startup capacitor (C_CPOUT) can be more lowered as the
discharge start voltage (V_CPOUT1) is higher, but note that if V_CPOUT1 > 2.0 V, the minimum operation
input voltage (V_IN min.) (the minimum input voltage value required for power to be output from the
OUT pin of the S-8880A Series) rises from 0.37 V to 0.39 V (Refer to Table 10).
3. When the capacitance of the startup capacitor (C_CPOUT) is lower, the discharge operation may start if
the ripple voltage of the CPOUT pin reaches the discharge start voltage (V_CPOUT1). The influence of the
ripple voltage can be reduced by setting C_CPOUT larger in this case.
4. Do not connect a load other than a capacitance to the CPOUT pin. Note that the discharge operation
may not be performed if a resistance, etc. is connected.
Discharge Start Voltage (V_CPOUT1)
1.8 V, 2.0 V
1.8 V to 2.4 V
Table 10
Operation Input Voltage (V_IN) Operation Temperature Range
0.37 V min.
Ta = −30°C to +60°C
0.39 V min.
Ta = −40°C to +85°C
Caution The above connection diagram and constant will not guarantee successful operation. Perform
thorough evaluation using the actual application to set the constant.
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