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ILC6377 데이터시트(PDF) 5 Page - Impala Linear Corporation |
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ILC6377 데이터시트(HTML) 5 Page - Impala Linear Corporation |
5 / 11 page 0.5A, 300kHz, SO-8 PWM/PFM Step Down Converter with Shutdown Impala Linear Corporation 5 (408) 574-3939 www.impalalinear.com June 1999 ILC6377 1.3 + V OUT ILC6377 (TOP VIE W) 3 45 1 2 6 7 8 + C L V IN S/D C SS SD1 L *C IN Fig 1. Typical step-down DC-DC converter application SD1 : MA735 Schottky Diode ( MATSUSHITA ) CL : 10V/47µF Tantalum Capacitor ( NICHICON, F93 ) CSS : 4700pF Ceramic Capacitor CIN : 16V / 10µF Tantalum Capacitor ( NICHICON, F93 ) 22 µH 47 µF 10 µF External component selection Proper selection of external components is important for achieving high performance. The output inductor selected should have low DC resistance on the order of 0.2W or less and saturation current rating of 1A or higher. Recommended inductors are Sumida CD54 (22mH, 0.18W max DC resistance) or Coilcraft DO3308P-223 (22mH, 0.18W max DC resistance) or equivalent. The catch diode should be a schottky diode with low for- ward drop and rated at 1A or greater current, MA735 or it’s equivalent is recommended. Input and output capacitors should be tantalum capacitors with low equivalent series resistance (ESR) and voltage rat- ing higher than the actual application. Soft-start Pin 4 of ILC6377 functions as the soft-start pin as well as the shutdown pin. A soft-start capacitor (from pin 4 to ground) controls the rate at which the power supply starts up; thus preventing large overshoots at the output as well as large in- rush current. The value for CSS should be 100pF or greater. Shutdown The ILC6377 is placed in shutdown mode by taking pin 4 to ground. In shutdown, the quiescent current of the device is under 2mA. When using the shutdown feature, pin 4 must be driven from an open collector or open drain output with- out employing an external pull-up resistor, as shown in fig- ure 2. Since pin 4 is also used to charge an external capac- itor for soft-start, this pin should not be driven from a push- pull CMOS type output. Over-current and short-circuit protection In the event of an over-current or short-circuit condition, the ILC6377 cycles the soft-start pin in a hiccup mode to provide fault protection. When the output voltage decreases due to overload, the ILC6377 will operate continuously at the max- imum duty cycle. If the period of maximum duty cycle oper- ation exceeds TPRO (typically 5 msec), pin 4 will be pulled low; thus discharging the external soft-start capacitor CSS. This action inhibits the regulator’s PWM action. Next, the ILC6377’s soft-start circuitry starts recharging CSS and initi- ates a controlled start-up. If the overload condition continues to exist, the above sequence of events will repeat; thus con- tinuing to cycle the soft-start function. Note that very little power is dissipated with this method of fault protection versus constant current limit protection. Even though the internal power MOSFET is pulsed on and off at high peak current, the DC current is low; thus leading to low power dissipation even under short-circuit conditions. Keep in mind that the duration of maximum duty cycle condition is used to trigger the ILC6377’s fault protec- tion circuit. As such, a small input-output (VIN - VOUT) differential voltage may trigger the device’s fault protec- tion circuitry even at low output current. Undervoltage Lockout The undervoltage lockout feature prevents faulty operation by disabling the operation of the regulator when input volt- age is below the minimum operating voltage, VUVLO. When the input voltage is lower than VUVLO the device disables the internal P-channel MOSFET and provides “high” output at both EXT1 and EXT2 outputs. Figure 2 shows a typical fixed output voltage step-down DC-DC converter application circuit for ILC6377SO-XX Fig. 1: Typical step-down wn DC-DC converter application |
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