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TS3003 bảng dữ liệu(PDF) 8 Page - Touchstone Semiconductor Inc |
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TS3003 bảng dữ liệu(HTML) 8 Page - Touchstone Semiconductor Inc |
8 / 10 page TS3003 Page 8 TS3003DS r1p0 RTFDS With an RSET = 4.32MΩ, the output frequency is approximately 25kHz with a 50% duty cycle. As design aids, Tables 1 lists TS3003 ’s typical O T for various standard values for RSET. The output frequency can be user-adjusted from 10kHz to 300kHz with a single resistor. The TS3003 also provides a separate PWM output signal at its PWMOUT terminal that is anti-phase with respect to FOUT. A dead time of approximately 106ns exists between FOUT and PWMOUT. To adjust the pulse width of the PWMOUT output, a single capacitor can be placed at the CPWM pin. To determine the capacitance needed for a desired pulse width, the following equation is to be used: C WM = ulse Width s x C WM VC WM 300mV Equation 2. CPWM Capacitor Calculation where ICPWM and VCPWM is the current supplied and voltage applied to the CPWM capacitor, respectively. The pulse width is determined based on the period of FOUT and should never be greater than the period at FOUT. Make sure the PWM_CNTRL pin is set to at least 400mV when calculating the pulse width of PWMOUT. Note VCPWM is approximately 300mV, which is the RSET voltage. Also note that ICPWM is approximately 1µA. The PWMOUT output pulse width can be adjusted further after selecting a CPWM capacitor. This can be achieved by applying a voltage to the PWM_CNTRL pin between VRSET and GND. With a voltage of at least VRSET, the pulse width is set based on Equation 2. For example, with a period of 40µs( 25kHz) a 47pF capacitor at the CPWM pin generates a pulse width of approximately 16µs. This can be calculated using equation 2. By reducing the PWM_CNTRL voltage from VRSET 300mV to GND, the pulse width is reduced from 16µs to approximately 8µs. This is a pulse width reduction of 50%. Note that as the FOUT frequency increases, the amount of pulse width reduction reduces and vice versa. Furthermore, if the PWMOUT output is half the frequency of the FOUT output, this means your CPWM capacitor is too large and as a result, the pulse width is greater than the FOUT period. In this case, use Equation 2 and reduce the capacitor value to less than the period. Connect CPWM to VDD to disable the PWM function and in turn, save power. Connect PWM_CNTRL to VDD for a fixed PWMOUT output pulse width, which is determined by the CPWM pin capacitor only. APPLICATIONS INFORMATION Minimizing Power Consumption To keep the TS3003 ’s power consumption low, resistive loads at the FOUT and PWMOUT terminals increase dc power consumption and therefore should be as large as possible. Capacitive loads at the FOUT and PWMOUT terminals increase the TS3003 ’s transient power consumption and, as well, should be as small as possible. One challenge to minimizing the TS3003 ’s transient power consumption is the probe capacitance of oscilloscopes and frequency counter instruments. Most instruments exhibit an input capacitance of 15pF or more. Unless buffered, the increase in transient load current can be as much as 400nA. To minimize capacitive loading, the technique shown in Figure 1 can be used. In this circuit, the principle of series-connected capacitors can be used to reduce the effective capacitive load at the TS3003 ’s O T and PWMOUT terminals. To determine the optimal value for CEXT once the probe capacitance is known by simply solving for CEXT using the following expression: or example, if the instrument’s input probe capacitance is 15pF and the desired effective load capacitance at either or both FOUT and PWMOUT terminals is to be ≤5p , then the value of C EXT should be ≤7.5p . Figure 1: Using an External Capacitor in Series with Probes Reduces Effective Capacitive Load. C T = 1 1 C OA 1 C RO Equation 3:External Capacitor Calculation |
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