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NE5561N Datasheet(PDF) 5 Page - NXP Semiconductors |
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NE5561N Datasheet(HTML) 5 Page - NXP Semiconductors |
5 / 5 page Philips Semiconductors Product specification NE/SE5561 Switched-mode power supply control circuit 1994 Aug 31 5 R VCC + V S * V CC (10 20mA) NOTE: See DC Electrical Characteristics for Current Fed VCC Range. NE5561 Voltage-Current-Fed Supply Characteristics Current-Fed Dropping Resistor VS RVCC VCC GND 1 8 mA 20 10 0 0 10 20 30 V SL00387 Figure 4. NE5561 Voltage-Current-Fed Supply Characteristics NE5561 START-UP The start-up, or initial turn-on, of this device requires some degree of external protective duty cycle limiting to prevent the duty cycle from initially going to the extreme maximum ( δ>90%). Either overcurrent limit or slow-start circuitry must be employed to limit duty cycle to a safe value during start-up. Both may be used, if desired. To implement slow-start, the start-up circuit can be used. The divider R1 and R2 sets a voltage, buffered by Q1, such that the output of the error amplifier is clamped to a maximum output voltage, thereby limiting the maximum duty cycle. The addition of capacitor C will cause this voltage to ramp-up slowly when power is applied, causing the duty cycle to ramp-up simultaneously. Overcurrent limit may be used also. To limit duty cycle in this mode, the switch current is monitored at Pin 6 and the output of the 5561 is disabled on a cycle-by-cycle basis when current reaches the programmed limit. With current limit control of slow-start, the duty cycle is limited to that value, just allowing maximum switch current to flow. (Approximately 0.50V measured at Pin 6.) APPLICATIONS 5V, 0.5A Buck Regulator Operates from 15V The converter design shows how simple it is to derive a TTL supply from a system supply of 15V (see Figure 1). The NE5561 drives a 2N4920 PNP transistor directly to provide switching current to the inductor. Overall line regulation is excellent and covers a range of 12V to 18V with minimal change (<10mV) in the output operating at full load. As with all NE5561 circuits, the auxiliary slow start and δMAX circuit is required, as evidenced by Q1. The δMAX limit may be calculated by using the relationship: R2 R1 ) R2 (8.2V) + V d MAX The maximum duty cycle is then determined from the pulse-width modulator transfer graph, with R1 and R2 being defined from the desired conditions. |
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