Traditional DC / DC voltage is generally the work of the more than 1.0 V, and if the input voltage dropped to 0.6 V below, DC / DC internal circuit can not work properly.

 In Figure 1 as an example, if the switch DC / DC drive voltage from the input supply in the past. 。 When the supply voltage lower than the DC / DC voltage when the driver, DC / DC will not be able to start.

Figure 1-driven input supply voltage from

 Well, if shown in Figure 2, the output of electricity from what is »

  Figure 2 drive voltage from the output voltage

 Similarly, when the supply voltage lower than the DC / DC drive voltage, DC / DC and simply can not start any step-up action.  However, if the DC / DC Once activated, the entire circuit will be a sustainable action.

 Boost the start-up circuit problem
 At this point in time, bring out another problem, that is such a low input voltage in the case of how to activate this a DC / DC? »

 At this time, we need to consider adding a start circuit, as shown in Figure 3.

 Figure 3 additional start-up IC boost the start-up circuit

 Seiko Electronics Co., Ltd. (SII) introduced the S-882Z series of pump products will be able to solve this problem.
 S-882Z series at the beginning voltage discharge size of four varieties: for 1.8 V, 2.0V, 2.2V and 2.4 V, the model suffix used to distinguish between 18,20,22 and 24.  For example, S-882Z20 discharge is the beginning of the voltage of 2.0 V charge pump.
 This series of major characteristics:

 ● input voltage VIN:
在Ta=-30～+60℃时为0.3～3.0V， In Ta =- 30 ~ +60 ℃ at 0.3 ~ 3.0 V,
在Ta=-40～+85℃时为0.35～3.0V； In Ta =- 40 ~ +85 ℃ at 0.35 ~ 3.0 V;
 ● work at the current consumption in the VIN = 0.3V at 0.5 mA (maximum);
 ● a close control, in closed or dormant state power consumption of less than 0.6 μ A (VIN = 0.3V);
 ● close control voltage discharge began to increase voltage 0.1 V (≤ 3.0 V);
； ● internal oscillator frequency of 350 kHz;
 ● external activated only by one capacitor (CCPOUT);
 ● small SOT-23-5 package;
 ● unleaded.

 S-882Z the internal structure shown in Figure 4.

 Figure 4 S-882Z internal structure diagram
 Below, we have to look at the S-882Z specific works (see Figure 5).

  Figure 5 S-882Z the working principle

 1 on the S-882Z series of VIN input voltage of 0.3 V and above, oscillation circuit can start work and from the oscillation circuit output CLK signals.

 2 through the CLK signal to drive the charge pump circuit, and will be in charge pump circuit VIN terminal voltage conversion to boost voltage.

 3 from the charge pump circuit boost output voltage, the charge will be slow to connect with CPOUT terminal with the start-up capacitor (CCPOUT), and therefore, CPOUT terminal voltage will slowly rise.
。 4 When CPOUT terminal voltage (VCPOUT) started to discharge voltage (VCPOUT1) above, the converters (COMP1) of the output signal will change from the high potential for low potential.  Therefore, in the "off" state of the discharge control switch (M1) will change to "open" status.

 5 M1 to "open" status after, CCPOUT charge of the premises will boost power from the terminal began OUT discharge.
 6 due to discharge, when VCPOUT to reduce the discharge stop voltage (VCPOUT2) when, M1 will change for the "off" state and stop the discharge.

 7 When the VM terminal voltage (VVM) reached on / off control voltage (VOFF) above, the converters (COMP2) of the output signal (EN-) will change from a low potential for high potential.  Therefore, the oscillation circuit will stop work and into dormant.

 8 When the VVM can not be achieved VOFF above, will make use of the circuit from the charge pump to boost the power of CCPOUT to recharge, and return to (3).

 S-882Z series are mainly used in solar batteries, fuel cells and other low-voltage power boost; RF tags internal voltage step-up (such as for highway toll system); work for intermittent power supply system.

 The biggest problem duty cycle MaxDuty

 The step-up circuit with ultra-low-input, in order to achieve a high output voltage, must have the support of the duty cycle.  Duty cycle (Duty) of the formula is: Duty = Ton / (Ton + Toff).

 In continuous current mode, the duty cycle (Duty) is calculated as the Duty = 1-Vin/Vout.。 According to this formula, if the importation of 0.5 V is sometimes a 5 V output boost circuit, the largest duty cycle to 90 percent, the average duty cycle for the step-up circuit of 80% to 90%, this is not fully meet the requirements of the .

 In this regard, we may consider the use of SII high-rate step-up DC / DC S-8337B, will be able to reach its maximum duty cycle of 94%. ； S-8337B the main features: input voltage of 1.8 ~ 6.0 V; reference voltage for the 1.0 ~ ± 1.5V; work for the current 0.5 mA (max); oscillation frequency of 47 ~ 200 kHz, in the external set the maximum duty cycle 75% to 94%, can also be set up outside; UVLO, soft start, external phase compensation settings; Action temperature range of -40 ~ +85 ℃; using TSSOP8 package.

 S-8337B diagram of the structure shown in Figure 6.

 Figure 6 S-8337B structure diagram

 A complete solution
 An ultra-low-voltage power management system needs to start circuitry and with the perfect boost circuit, with S-882Z series with S-8337B series will be able to achieve this objective. Figure 7 for the use of the S-882Z24 SII and the S-8337BAJA a low input voltage step-up circuit, Figure 8 for the 0.5 V input circuit in case the efficiency curve.

 Figure 7 S-882Z24 + S-8337BAJA a low input voltage step-up circuit

 Figure 8 0.5V under the circuit efficiency curve

 From can be seen in Figure 8, 10 times the boost in this situation, and the need to reach 200 mA output current, the output efficiency of the circuit can still achieve more than 80 percent, which is already a breakthrough with the programme.