Datasheet LTC6906 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Micropower, 10kHz to 1MHz Resistor Set Oscillator in SOT-23 |
| Pages / Page | 14 / 8 — APPLICATIONS INFORMATION Selecting RSET and the Divider Ratio. For any … |
| File Format / Size | PDF / 172 Kb |
| Document Language | English |
APPLICATIONS INFORMATION Selecting RSET and the Divider Ratio. For any given
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LTC6906
APPLICATIONS INFORMATION Selecting RSET and the Divider Ratio
10000 The LTC6906 contains a master oscillator followed by a digital divider (see the Block Diagram). RSET determines the master oscillator frequency and the DIV pin sets the 10 3 1 1000 divider ratio, N. The range of frequencies accessible in (kΩ) each divider ratio overlap, as shown in Figure 7. This figure SETR is derived from the equations in Table 1.
For any given
100
frequency, power can be minimized by minimizing the master oscillator frequency. This implies maximizing RSET and using the lowest possible divider ratio, N
. The 10 relationship between R 1 10 100 1000 10000 SET, N and the unloaded power OUTPUT FREQUENCY (kHz) consumption is shown in Figure 8, where we can clearly 6906 F07 see that supply current decreases for large values of RSET.
Figure 7. RSET vs Desired Output Frequency (LTC6906)
For a discussion of jitter and divide ratio, refer to page 11. 80
Minimizing Power Consumption
CLOAD = 0 70 V+ = 3V The supply current of the LTC6906 has four current TA = 25°C 60 components: 50 • Constant (Independent V+, ƒ (A) OUT and CLOAD) 40 • Proportional to I I SUPPLY SET (which is the current in RSET) 30 • Proportional to V+, ƒ 20 OUT and CLOAD 10 • Proportional to V+ and RLOAD 0 An approximate expression for the total supply current is: 100 1000 RSET (kΩ) 6906 F08 I+ ≅ 5μA + 6 • I + ( + )+ V+ SET V+ • ƒOUT • CLOAD 5pF 2 • R
Figure 8. Unloaded Supply Current vs R
LOAD
SET
V ≅ 5μA + 6 • SET + V+ • ƒ ( + )+ V+ OUT • CLOAD 5pF 80 RSET 2 • RLOAD V+ = 2.7V 70 VSET is approximately 650mV at 25°C, but varies with 60 temperature. This behavior is shown in the Typical Per- 1 50 formance Characteristics section. 3 40 10 Power can be minimized by maximizing RSET, minimizing 30 the load on the OUT pin and operating at lower frequen- 20 cies. Figure 9 shows total supply current vs frequency POWER SUPPLY CURRENT (A) under typical conditions. Below 100kHz the load current 10 is negligible for the 5pF load shown. 0 0 200 400 600 800 1000 1200 MASTER OSCILLATOR FREQUENCY (kHz) 6906 F09
Figure 9. Supply Current vs Frequency
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APPLICATIONS INFORMATION Selecting RSET and the Divider Ratio
10000 The LTC6906 contains a master oscillator followed by a digital divider (see the Block Diagram). RSET determines the master oscillator frequency and the DIV pin sets the 10 3 1 1000 divider ratio, N. The range of frequencies accessible in (kΩ) each divider ratio overlap, as shown in Figure 7. This figure SETR is derived from the equations in Table 1.
For any given
100
frequency, power can be minimized by minimizing the master oscillator frequency. This implies maximizing RSET and using the lowest possible divider ratio, N
. The 10 relationship between R 1 10 100 1000 10000 SET, N and the unloaded power OUTPUT FREQUENCY (kHz) consumption is shown in Figure 8, where we can clearly 6906 F07 see that supply current decreases for large values of RSET.
Figure 7. RSET vs Desired Output Frequency (LTC6906)
For a discussion of jitter and divide ratio, refer to page 11. 80
Minimizing Power Consumption
CLOAD = 0 70 V+ = 3V The supply current of the LTC6906 has four current TA = 25°C 60 components: 50 • Constant (Independent V+, ƒ (A) OUT and CLOAD) 40 • Proportional to I I SUPPLY SET (which is the current in RSET) 30 • Proportional to V+, ƒ 20 OUT and CLOAD 10 • Proportional to V+ and RLOAD 0 An approximate expression for the total supply current is: 100 1000 RSET (kΩ) 6906 F08 I+ ≅ 5μA + 6 • I + ( + )+ V+ SET V+ • ƒOUT • CLOAD 5pF 2 • R
Figure 8. Unloaded Supply Current vs R
LOAD
SET
V ≅ 5μA + 6 • SET + V+ • ƒ ( + )+ V+ OUT • CLOAD 5pF 80 RSET 2 • RLOAD V+ = 2.7V 70 VSET is approximately 650mV at 25°C, but varies with 60 temperature. This behavior is shown in the Typical Per- 1 50 formance Characteristics section. 3 40 10 Power can be minimized by maximizing RSET, minimizing 30 the load on the OUT pin and operating at lower frequen- 20 cies. Figure 9 shows total supply current vs frequency POWER SUPPLY CURRENT (A) under typical conditions. Below 100kHz the load current 10 is negligible for the 5pF load shown. 0 0 200 400 600 800 1000 1200 MASTER OSCILLATOR FREQUENCY (kHz) 6906 F09
Figure 9. Supply Current vs Frequency
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