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LMZ12002TZE-ADJ bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LMZ12002TZE-ADJ bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
11 / 18 page General Description The LMZ12002 SIMPLE SWITCHER® power module is an easy-to-use step-down DC-DC solution capable of driving up to 2A load with exceptional power conversion efficiency, line and load regulation, and output accuracy. The LMZ12002 is available in an innovative package that enhances thermal performance and allows for hand or machine soldering. The LMZ12002 can accept an input voltage rail between 4.5V and 20V and deliver an adjustable and highly accurate output voltage as low as 0.8V. The LMZ12002 only requires three external resistors and four external capacitors to complete the power solution. The LMZ12002 is a reliable and robust design with the following protection features: thermal shutdown, in- put under-voltage lockout, output over-voltage protection, short-circuit protection, output current limit, and allows startup into a pre-biased output. A single resistor adjusts the switch- ing frequency up to 1 MHz. COT Control Circuit Overview Constant On Time control is based on a comparator and an on-time one shot, with the output voltage feedback compared with an internal 0.8V reference. If the feedback voltage is be- low the reference, the main MOSFET is turned on for a fixed on-time determined by a programming resistor R ON. RON is connected to V IN such that on-time is reduced with increasing input supply voltage. Following this on-time, the main MOS- FET remains off for a minimum of 260 ns. If the voltage on the feedback pin falls below the reference level again the on-time cycle is repeated. Regulation is achieved in this manner. Design Steps for the LMZ12002 Application The LMZ12002 is fully supported by Webench® and offers the following: Component selection, electrical and thermal simulations as well as the build-it board for a reduction in de- sign time. The following list of steps can be used to manually design the LMZ12002 application. • Select minimum operating V IN with enable divider resistors • Program V O with divider resistor selection • Program turn-on time with soft-start capacitor selection • Select C O • Select C IN • Set operating frequency with R ON • Determine module dissipation • Layout PCB for required thermal performance ENABLE DIVIDER, R ENT AND RENB SELECTION The enable input provides a precise 1.18V band-gap rising threshold to allow direct logic drive or connection to a voltage divider from a higher enable voltage such as Vin. The enable input also incorporates 90 mV (typ) of hysteresis resulting in a falling threshold of 1.09V. The maximum recommended voltage into the EN pin is 6.5V. For applications where the midpoint of the enable divider exceeds 6.5V, a small zener can be added to limit this voltage. The function of this resistive divider is to allow the designer to choose an input voltage below which the circuit will be dis- abled. This implements the feature of programmable under voltage lockout. This is often used in battery powered systems to prevent deep discharge of the system battery. It is also useful in system designs for sequencing of output rails or to prevent early turn-on of the supply as the main input voltage rail rises at power-up. Applying the enable divider to the main input rail is often done in the case of higher input voltage sys- tems where a lower boundary of operation should be estab- lished. In the case of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power- up cycle than the LMZ12002 output rail. The two resistors should be chosen based on the following ratio: R ENT / RENB = (VIN UVLO / 1.18V) – 1 (1) The LMZ12002 demonstration and evaluation boards use 11.8k Ω for R ENB and 32.4kΩ for RENT resulting in a rising UV- LO of 4.5V. This divider presents 5.34V to the EN input when the divider input is raised to 20V. The EN pin is internally pulled up to VIN and can be left float- ing for always-on operation. OUTPUT VOLTAGE SELECTION Output voltage is determined by a divider of two resistors connected between V O and ground. The midpoint of the di- vider is connected to the FB input. The voltage at FB is compared to a 0.8V internal reference. In normal operation an on-time cycle is initiated when the voltage on the FB pin falls below 0.8V. The main MOSFET on-time cycle causes the output voltage to rise and the voltage at the FB to exceed 0.8V. As long as the voltage at FB is above 0.8V, on-time cycles will not occur. The regulated output voltage determined by the external di- vider resistors R FBT and RFBB is: V O = 0.8V * (1 + RFBT / RFBB) (2) Rearranging terms; the ratio of the feedback resistors for a desired output voltage is: R FBT / RFBB = (VO / 0.8V) - 1 (3) These resistors should be chosen from values in the range of 1.0 kohm to 10.0 kohm. For V O = 0.8V the FB pin can be connected to the output di- rectly so long as an output preload resistor remains that draws more than 20uA. Converter operation requires this minimum load to create a small inductor ripple current and maintain proper regulation when no load is present. A feed-forward capacitor is placed in parallel with R FBT to im- prove load step transient response. Its value is usually deter- mined experimentally by load stepping between DCM and CCM conduction modes and adjusting for best transient re- sponse and minimum output ripple. A table of values for R FBT , RFBB , CFF and RON is included in the applications schematic. SOFT-START CAPACITOR SELECTION Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time to prevent overshoot. Upon turn-on, after all UVLO conditions have been passed, an internal 8uA current source begins charging the external soft-start capacitor. The soft-start time duration to reach steady state operation is given by the formula: t SS = VREF * CSS / Iss = 0.8V * CSS / 8uA (4) This equation can be rearranged as follows: C SS = tSS * 8 μA / 0.8V (5) Use of a 0.022 μF capacitor results in 2.2 msec soft-start du- ration. This is recommended as a minimum value. As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. The soft-start capacitor continues 11 www.national.com |
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