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NCP5007SNT1 Datasheet(PDF) 6 Page - ON Semiconductor |
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NCP5007SNT1 Datasheet(HTML) 6 Page - ON Semiconductor |
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6 / 22 page  NCP5007 http://onsemi.com 6 Basically, the chip operates with two cycles: Cycle #1 : time t1, the energy is stored into the inductor Cycle #2 : time t2, the energy is dumped to the load The POR signal sets the flip−flop and the first cycle takes place. When the current hits the peak value, defined by the error amplifier associated with the loop regulation, the flip−flop resets, the NMOS is deactivated and the current is dumped into the load. Since the timing is application dependent, the internal timer limits the Toff cycle to 320 ns (typical), making sure the system operates in a continuous mode to maximize the energy transfer. Figure 4. Basic DC−DC Operation First Startup Normal Operation IL 0 mA Ids 0 mA Io 0 mA Iv Ipeak t t t t1 t2 Based on the data sheet, the current flowing into the inductor is bounded by two limits: • Ipeak Value: Internally fixed to 350 mA typical • Iv Value: Limited by the fixed Toff time built in the chip (320 ns typical) The system operates in a continuous mode as depicted in Figure 4 and t1 & t2 times can be derived from basic equations. (Note: The equations are for theoretical analysis only, they do not include the losses.) E + L* di dt (eq. 1) Let E = Vbat, then: t1 + (Ip * Iv) * L Vbat (eq. 2) t2 + (Ip * Iv) * L Vo * Vbat (eq. 3) Since t2 = 320 ns typical and Vo = 22 V maximum, then (assuming a typical Vbat = 3.0 V): DI + t2 * (Vo * Vbat) L (eq. 4) DImax + 320e * 9 *(22 * 3.0) 22e * 6 + 276 mA Of course, from a practical stand point, the inductor must be sized to cope with the peak current present in the circuit to avoid saturation of the core. On top of that, the ferrite material shall be capable to operate at high frequency (1.0 MHz) to minimize the Foucault’s losses developed during the cycles. The operating frequency can be derived from the electrical parameters. Let V = Vo − Vbat, rearranging Equation 1: ton + dI * L E (eq. 5) Since toff is nearly constant (according to the 320 ns typical time), the dI is constant for a given load and inductance value. Rearranging Equation 5 yields: ton + V*dt L *L E (eq. 6) Let E = Vbat, and Vopk = output peak voltage, then: ton + (Vopk * Vbat) * dt Vbat (eq. 7) Finally, the operating frequency is: F + 1 ton ) toff (eq. 8) The output power supplied by the NCP5007 is limited to one watt: Figure 5 shows the maximum power that can be delivered by the chip as a function of the input voltage. |
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