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DS1227 Datasheet(PDF) 9 Page - Dallas Semiconductor |
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DS1227 Datasheet(HTML) 9 Page - Dallas Semiconductor |
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9 / 20 page  DS1227 022698 9/20 BOOST MODE OPERATION The DS1227 Kickstarter incorporates all of the neces- sary control and power switching functions required for its +3V to +5V step-up DC-to-DC converter. These func- tions include a bandgap reference, oscillator, voltage comparator, catch diode and an N-channel MOSFET. The only external components required are an output fil- ter capacitor and a low cost inductor. The block diagram shown in Figure 7 illustrates the DC-to-DC converter. When kickstarting occurs from an initial powered down state (i.e., VCCO1, VCCO2, and VCCO3 turned off), an in- ternal start sequence is initiated within the DS1227. During this sequence, the VCCO1 output remains shut off and the BOOST pin is sampled in order to determine if the DS1227 is configured for boost mode operation. If BOOST is low, then boost mode operation is enabled and the DC-to-DC converter is started. The internal DC-to-DC converter is started by enabling the on-chip 40 KHz oscillator. It then begins to build up the voltage on the VDCO filter capacitor. Internal counter logic insures that the DC-to-DC converter stays in start mode for a minimum of six clock periods (nominally 150 µs @ 40 KHz). After this initial delay time, the VDCO output is monitored by the internal Error Comparator as it slews up to VDCON. As long as the VDCO voltage re- mains below the preset value, the Error Comparator will be switched high and the internal 40 KHz oscillator will be connected to the gate of the VDCI driver. The VDCI driver is a large N-channel MOSFET with a typical ON resistance of less than 4 Ohms and is capa- ble of supplying a peak current of 450 mA. The output device is turned on during each ON half-cycle gener- ated by the internal square-wave oscillator, and is turned off during each OFF half-cycle. During each ON half-cycle, the current through the inductor rises linearly, storing energy in the coil. When the output device is turned off, the external inductor’s magnetic field col- lapses, and the voltage across the inductor reverses sign. The voltage at VDCI then rises until the internal diode is forward biased, delivering power to the VDCO output. The converter is thereby powered from its own VDCO output. This is often referred to as “bootstrapped” operation, since the circuit figuratively “lifts” itself up. In order to guarantee that the Kickstarter can bootstrap it- self up to operating voltage, the VDCI voltage must be at the minimum level of VDCISU as listed in the DC charac- teristics section of this data sheet. When the voltage on VDCO rises to the VDCON thresh- old, the internal signal called “HIENUF” will be active and the VCCO1 PMOS device is switched on. As noted above, internal circuitry insures that this device will not be switched on for a minimum of 6 clock cycles from the time that the DC-to-DC converter is started. However, since the recommended values for the external LC com- ponents result in a time constant which is much longer than six cycles, the actual slew rate will in practice be much longer than this delay time. If loading of the VCCO outputs causes VDCO to drop be- low VDCOFF the DS1227 will deactivate HIENUF and the VCCO1 PMOS device as well as the other VCCO PMOS devices will be switched off. The VDCO voltage will then be monitored for the VDCON trip point before re- connecting the load. As a result, the power control regu- lation loop could oscillate between these two states until the VCCO1 node had sufficient charge to remain above the VDCOFF threshold. To prevent this from occurring, the value of the filter capacitor must be sufficiently large. For large capacitive loads on VCCO1 the output may dip below VDCOFF as a result of charge sharing and a larger regulation capacitor at VDCO may be required. For large resistive loads the inductance and capacitance values may need to be adjusted using a smaller inductor value and large capacitance. In order not to violate the peak VDCI current it may be necessary to use the external os- cillator OSCEXT to drive an additional switchmode boost regulator, as shown in Figure 8. Following the above described start sequence, normal boost operation is performed by the converter. VDCO output voltage is constantly monitored by the error com- parator. When VDCO voltage drops below the preset val- ue, the error comparator switches high and connects the internal 40 KHz oscillator to the gate of the VDCI out- put driver. When the output voltage reaches the desired level, the error comparator inhibits the VDCI output driv- er until the load on VCCO1 discharges the output filter ca- pacitor to less than the desired output level. INDUCTOR SELECTION The available output current from the Kickstarter’s on-chip DC-DC boost converter is a function of the input voltage, external inductor value, output voltage and the operating frequency. For most applications, the inductor is the only design variable since the internal oscillator is preset to a fixed value of 40 KHz. The proper inductor must have the following characteristics: |
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