Datasheet NCP1421 (ON Semiconductor) - 9

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NCP1421 DETAILED OPERATION DESCRIPTION
NCP1421 is a monolithic micropower high−frequency of dead time is introduced to make sure M1 is completely step−up voltage switching converter IC specially designed turned OFF before M2 is being turned ON. for battery operated hand−held electronic products up to The previously mentioned situation occurs when the 600 mA loading. It integrates a Synchronous Rectifier to regulator is operating in CCM, M2 is being turned OFF, M1 improve efficiency as well as to eliminate the external is just turned ON, and M2 is not being completely turned Schottky diode. High switching frequency (up to 1.2 MHz) OFF. A dead time is also needed to make sure M2 is allows for a low profile inductor and output capacitor to be completely turned OFF before M1 is being turned ON. used. Low−Battery Detector, Logic−Controlled Shutdown, As coil current is dropped to zero when the regulator is and Cycle−by−Cycle Current Limit provide value−added operating in DCM, M2 should be OFF. If this does not features for various battery−operated applications. With all occur, the reverse current flows from the output bulk these functions ON, the quiescent supply current is capacitor through M2 and the inductor to the battery input, typically only 8.5 A. This device is available in a compact causing damage to the battery. The ZLC comparator comes Micro8 package. with fixed offset voltage to switch M2 OFF before any reverse current builds up. However, if M2 is switched OFF
PFM Regulation Scheme
too early, large residue coil current flows through the body From the simplified functional diagram (Figure 1), the diode of M2 and increases conduction loss. Therefore, output voltage is divided down and fed back to pin 1 (FB). determination of the offset voltage is essential for optimum This voltage goes to the non−inverting input of the PFM performance. With the implementation of the synchronous comparator whereas the comparator’s inverting input is rectification scheme, efficiency can be as high as 94% with connected to the internal voltage reference, REF. A this device. switching cycle is initiated by the falling edge of the comparator, at the moment the main switch (M1) is turned
Cycle−by−Cycle Current Limit
ON. After the maximum ON−time (typically 0.72 S) In Figure 1, a SENSEFET is used to sample the coil elapses or the current limit is reached, M1 is turned OFF current as M1 is ON. With that sample current flowing and the synchronous switch (M2) is turned ON. The M1 through a sense resistor, a sense−voltage is developed. The OFF time is not less than the minimum OFF−time threshold detector (ILIM) detects whether the (typically 0.12 S), which ensures complete energy sense−voltage is higher than the preset level. If the sense transfer from the inductor to the output capacitor. If the voltage is higher than the present level, the detector output regulator is operating in Continuous Conduction Mode notifies the Control Logic to switch OFF M1, and M1 can (CCM), M2 is turned OFF just before M1 is supposed to be only be switched ON when the next cycle starts after the ON again. If the regulator is operating in Discontinuous minimum OFF−time (typically 0.12 S). With proper Conduction Mode (DCM), which means the coil current sizing of the SENSEFET and sense resistor, the peak coil will decrease to zero before the new cycle starts, M1 is current limit is typically set at 1.5 A. turned OFF as the coil current is almost reaching zero. The comparator (ZLC) with fixed offset is dedicated to sense
Voltage Reference
the voltage drop across M2 as it is conducting; when the The voltage at REF is typically set at 1.20 V and can voltage drop is below the offset, the ZLC comparator output up to 2.5 mA with load regulation ±2% at VOUT output goes HIGH and M2 is turned OFF. Negative equal to 3.3 V. If VOUT is increased, the REF load feedback of closed−loop operation regulates voltage at capability can also be increased. A bypass capacitor of pin 1 (FB) equal to the internal reference voltage (1.20 V). 200 nF is required for proper operation when REF is not loaded. If REF is loaded, a 1.0 F capacitor at the REF pin
Synchronous Rectification
is needed. The Synchronous Rectifier is used to replace the Schottky Diode to reduce the conduction loss contributed
True−Cutoff
by the forward voltage of the Schottky Diode. The The NCP1421 has a True−Cutoff function controlled by Synchronous Rectifier is normally realized by powerFET the multi−function pin LBI/EN (pin 2). Internal circuitry with gate control circuitry that incorporates relatively can isolate the current through the body diode of switch M2 complicated timing concerns. to load. Thus, it can eliminate leakage current from the As the main switch (M1) is being turned OFF and the battery to load in shutdown mode and significantly reduce synchronous switch M2 is just turned ON with M1 not battery current consumption during shutdown. The being completely turned OFF, current is shunt from the shutdown function is controlled by the voltage at pin 2 output bulk capacitor through M2 and M1 to ground. This (LBI/EN). When pin 2 is pulled to lower than 0.3 V, the power loss lowers overall efficiency and possibly damages controller enters shutdown mode. In shutdown mode, when the switching FETs. As a general practice, a certain amount switches M1 and M2 are both switched OFF, the internal
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