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LM2695SDX Arkusz danych(PDF) 8 Page - National Semiconductor (TI) |
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LM2695SDX Arkusz danych(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Functional Description The LM2695 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying 1.25A to the load. This high voltage regulator contains a 33V N-Channel buck switch, is easy to implement, and is available in the thermally enhanced LLP-10 and TSSOP-14EP packages. The regula- tor’s operation is based on a hysteretic control scheme, and uses an on-time control which varies inversely with V IN. This feature allows the operating frequency to remain relatively constant with load and input voltage variations. The hyster- etic control requires no loop compensation resulting in very fast load transient response. The valley current limit detec- tion circuit, internally set at 1.25A, holds the buck switch off until the high current level subsides. The functional block diagram is shown in Figure 3. The LM2695 can be applied in numerous applications to efficiently regulate down higher voltages. Additional features include: Thermal shutdown, V CC under-voltage lockout, gate drive under-voltage lockout, and maximum duty cycle limiter. Hysteretic Control Circuit Overview The LM2695 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an inter- nal reference (2.5V). If the FB voltage is below the reference the buck switch is turned on for a time period determined by the input voltage and a programming resistor (R ON). Follow- ing the on-time the switch remains off for a minimum of 250 ns, and until the FB voltage falls below the reference. The buck switch then turns on for another on-time period. Typi- cally, during start-up, or when the load current increases suddenly, the off-times are at the minimum of 250 ns. Once regulation is established, the off-times are longer. When in regulation, the LM2695 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous con- duction mode current always flows through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The operating frequency is approximately: (1) The buck switch duty cycle is equal to: (2) In discontinuous conduction mode current through the induc- tor ramps up from zero to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the reference - until then the inductor current remains zero, and the load current is supplied by the output capacitor (C2). In this mode the operating frequency is lower than in continu- ous conduction mode, and varies with load current. Conver- sion efficiency is maintained at light loads since the switch- ing losses reduce with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: (3) where R L = the load resistance. The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as follows: V OUT =2.5x(R1+R2)/R2 Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. The LM2695 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Figure 3). For applications where lower output voltage ripple is re- quired the output can be taken directly from a low ESR output capacitor as shown in Figure 4. However, R3 slightly degrades the load regulation. 20170410 FIGURE 4. Low Ripple Output Configuration www.national.com 8 |
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