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TS3004 Datasheet(PDF) 8 Page - Silicon Laboratories |
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TS3004 Datasheet(HTML) 8 Page - Silicon Laboratories |
8 / 12 page TS3004 Page 8 TS3004 Rev. 1.0 With an RSET = 4.32M Ω and FDIV2:0=111, the FOUT period is approximately 83.89s with a 50% duty cycle. As design aids, Tables 2 lists TS3004’s typical FOUT period for various standard values for RSET and FDIV2:0 = 111(7). The output period can be user-adjusted from 3.3µs to 233s without additional components. Frequency divider inputs FDIV2:0 can be set to a logic state HIGH or LOW in order to set the desired frequency as shown in to Table 1. The TS3004 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: CPWM(F)= Pulse Width(s) x ICPWM VCPWM≅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 either 1µA or 100nA. Refer to Table 1. 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 TS3004’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 TS3004’s transient power consumption and, as well, should be as small as possible. One challenge to minimizing the TS3004’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 TS3004’s FOUT 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: Figure 1: Using an External Capacitor in Series with Probes Reduces Effective Capacitive Load. CEXT = 1 1 CLOAD(EFF) - 1 CPROBE Equation 3:External Capacitor Calculation |
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