Datasheet LT1795 (Analog Devices) - 8
| Fabricante | Analog Devices |
| Descripción | Dual 500mA/50MHz Current Feedback Line Driver Amplifier |
| Páginas / Página | 12 / 8 — APPLICATIO S I FOR ATIO. Figure 5. Setting Amplifier Supply Current Level … |
| Formato / tamaño de archivo | PDF / 158 Kb |
| Idioma del documento | Inglés |
APPLICATIO S I FOR ATIO. Figure 5. Setting Amplifier Supply Current Level with ON/OFF Control, Version 1
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LT1795
U U W U APPLICATIO S I FOR ATIO
quiescent current to 33mA with V Figure 8 illustrates a partial shutdown with direct logic S = ±15V. If ON/OFF control is desired in addition to reduced quiescent current, control. By keeping the output stage slightly biased on, the then the circuits in Figures 5 to 7 can be employed. output impedance remains low, preserving the line termi- nation. The design equations are: V+ R 115 • V SHDN R H 1= RB 10 SHDN INTERNAL ( ) –( ) OFF I I 10k S S LOGIC THRESHOLD ON OFF Q1 ~1.4V 11 SHDNREF ON (0V) (3.3V/5V) 115 • V – V ( ) CC SHDN 1795 F05 Q1: 2N3904 OR EQUIVALENT R2 = V / V • I – I I ( ) ( ) ( ) SHDN H S S S ON OFF + ( )OFF
Figure 5. Setting Amplifier Supply Current Level with ON/OFF Control, Version 1
where V+ VH = Logic High Level 10 SHDN (IS)ON = Supply Current Fully On RPULLUP >500k (IS)OFF = Supply Current Partially On 11 SHDNREF VSHDN = Shutdown Pin Voltage ≈1.4V R R SHDN1 SHDN2 VCC = Positive Supply Voltage R R B1 B2 ON ON 10k 10k Q1A Q1B OFF OFF VCC (0V) (0V) (3.3V/5V) (3.3V/5V) R2 ON 1795 F06 R1 Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL RB) OFF 10 INTERNAL (0V) SHDN LOGIC THRESHOLD ISY ~ 1.4V (3.3V/5V) CONTROL
Figure 6. Setting Multiple Amplifier Supply Current Levels with ON/OFF Control, Version 2
11 SHDNREF 1795 F08
Figure 8. Partial Shutdown
ON R SHDN EXT OFF 10 INTERNAL LOGIC THRESHOLD (0V) IPROG ISY ~ 1.4V
THERMAL CONSIDERATIONS
(3.3V/5V) CONTROL IPROG ≅ 0.5mA SHDNREF FOR R The LT1795 contains a thermal shutdown feature that EXT = 0Ω 11 (SEE SHDN PIN protects against excessive internal (junction) temperature. CURRENT vs 1795 F07 VOLTAGE If the junction temperature of the device exceeds the CHARACTERISTIC) protection threshold, the device will begin cycling between normal operation and an off state. The cycling is not
Figure 7. Setting Amplifier Supply Current Level
harmful to the part. The thermal cycling occurs at a slow
with ON/OFF Control, Version 3
rate, typically 10ms to several seconds, which depends on the power dissipation and the thermal time constants of the package and heat sinking. Raising the ambient tempera- 1795fa 8
U U W U APPLICATIO S I FOR ATIO
quiescent current to 33mA with V Figure 8 illustrates a partial shutdown with direct logic S = ±15V. If ON/OFF control is desired in addition to reduced quiescent current, control. By keeping the output stage slightly biased on, the then the circuits in Figures 5 to 7 can be employed. output impedance remains low, preserving the line termi- nation. The design equations are: V+ R 115 • V SHDN R H 1= RB 10 SHDN INTERNAL ( ) –( ) OFF I I 10k S S LOGIC THRESHOLD ON OFF Q1 ~1.4V 11 SHDNREF ON (0V) (3.3V/5V) 115 • V – V ( ) CC SHDN 1795 F05 Q1: 2N3904 OR EQUIVALENT R2 = V / V • I – I I ( ) ( ) ( ) SHDN H S S S ON OFF + ( )OFF
Figure 5. Setting Amplifier Supply Current Level with ON/OFF Control, Version 1
where V+ VH = Logic High Level 10 SHDN (IS)ON = Supply Current Fully On RPULLUP >500k (IS)OFF = Supply Current Partially On 11 SHDNREF VSHDN = Shutdown Pin Voltage ≈1.4V R R SHDN1 SHDN2 VCC = Positive Supply Voltage R R B1 B2 ON ON 10k 10k Q1A Q1B OFF OFF VCC (0V) (0V) (3.3V/5V) (3.3V/5V) R2 ON 1795 F06 R1 Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL RB) OFF 10 INTERNAL (0V) SHDN LOGIC THRESHOLD ISY ~ 1.4V (3.3V/5V) CONTROL
Figure 6. Setting Multiple Amplifier Supply Current Levels with ON/OFF Control, Version 2
11 SHDNREF 1795 F08
Figure 8. Partial Shutdown
ON R SHDN EXT OFF 10 INTERNAL LOGIC THRESHOLD (0V) IPROG ISY ~ 1.4V
THERMAL CONSIDERATIONS
(3.3V/5V) CONTROL IPROG ≅ 0.5mA SHDNREF FOR R The LT1795 contains a thermal shutdown feature that EXT = 0Ω 11 (SEE SHDN PIN protects against excessive internal (junction) temperature. CURRENT vs 1795 F07 VOLTAGE If the junction temperature of the device exceeds the CHARACTERISTIC) protection threshold, the device will begin cycling between normal operation and an off state. The cycling is not
Figure 7. Setting Amplifier Supply Current Level
harmful to the part. The thermal cycling occurs at a slow
with ON/OFF Control, Version 3
rate, typically 10ms to several seconds, which depends on the power dissipation and the thermal time constants of the package and heat sinking. Raising the ambient tempera- 1795fa 8