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LTC1649CS Datasheet(PDF) 11 Page - Linear Technology |
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LTC1649CS Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LTC1649 The CPOUT pin can typically supply 50mA at 5V, adequate to power the VCC and PVCC pins. This supply can also be used to power external circuitry, but any additional current drawn from CPOUT subtracts from the current available to drive the external MOSFETs. Circuits with small external MOSFETs can draw as much as 20mA or 30mA from CPOUT without hindering performance. High output cur- rent circuits with large or multiple external MOSFETs may need every milliamp they can get from CPOUT, and external loads should be minimized. The charge pump at PVCC1 is more limited in its abilities, and should not be connected to anything except PVCC1. In particular, do not connect a bypass capacitor from PVCC1 to ground—it will steal charge from the charge pump and actually degrade perfor- mance. Compensation and Transient Response The LTC1649 voltage feedback loop is compensated at the COMP pin; this is the output node of the internal gm error amplifier. The loop can generally be compensated prop- erly with an RC network from COMP to GND and an additional small C from COMP to GND (Figure 5). Loop stability is affected by inductor and output capacitor values and by other factors. Optimum loop response can be obtained by using a network analyzer to find the loop poles and zeros; nearly as effective and a lot easier is to empirically tweak the RC values until the transient recovery looks right with an output load step. Output transient response is set by three major factors: the time constant of the inductor and the output capacitor, the ESR of the output capacitor, and the loop compensation components. The first two factors usually have much more impact on overall transient recovery time than the third; unless the loop compensation is way off, more improvement can be had by optimizing the inductor and the output capacitor than by fiddling with the loop com- pensation components. In general, a smaller value induc- tor will improve transient response at the expense of ripple and inductor core saturation rating. Minimizing output capacitor ESR will also help optimize output transient response. See Input and Output Capacitors for more information. Soft Start and Current Limit The LTC1649 includes a soft start circuit at the SS pin; this circuit is used both for initial start-up and during current limit operation. SS requires an external capacitor to GND with the value determined by the required soft start time. An internal 12 µA current source is included to charge the external capacitor. Soft start functions by clamping the maximum voltage that the COMP pin can swing to, thereby controlling the duty cycle (Figure 6). The LTC1649 will begin to operate at low duty cycle as the SS pin rises to about 2V below the VCC pin. As SS continues to rise, the duty cycle will increase until the error amplifier takes over and begins to regulate the output. When SS reaches 1V below VCC the LTC1649 will be in full operation. An internal switch shorts the SS pin to GND during shutdown. APPLICATIONS INFORMATION LTC1649 COMP 1659 F05 C1 CC RC Figure 5. Compensation Pin Hook-Up Figure 6. Soft Start Clamps COMP Pin 1649 F06 CSS COMP SS LTC1649 12 µA VCC FB |
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