Datasheet LT1939 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller |
| Pages / Page | 24 / 10 — OPERATION. APPLICATIONS INFORMATION Choosing the Output Voltage. Choosing … |
| File Format / Size | PDF / 358 Kb |
| Document Language | English |
OPERATION. APPLICATIONS INFORMATION Choosing the Output Voltage. Choosing the Switching Frequency
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LT1939
OPERATION
a drive voltage higher than VIN to saturate the output NPN A power good comparator with 30mV of hysteresis trips and maintain high effi ciency. when both FB and LFB are above 90% of the 0.8V refer- ence. The PG output is an open collector NPN that is off In addition to the switching regulator, the LT1939 contains when the output is in regulation allowing a resistor to pull a NPN linear regulator with a 0.8V reference, and 13mA the PG pin to a desired voltage. The PG output is an open- current capability. The 0.8 reference will track the SS pin collector NPN that is on when the output is in regulation in the same manner as the switching regulator. The linear providing either drive for an output disconnect transistor output can also be confi gured to drive an external NPN to or inverted power good logic. provide a linear regulator with higher current capability.
APPLICATIONS INFORMATION Choosing the Output Voltage
maximum recommended frequency can be approximated by the equation: The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- 1 Frequency (Hz)= VOUT1 + VD • tors according to: V t IN VSW + VD ON(MIN) V R1= R2 OUT1 – 1 0.8V where VD is the forward voltage drop of the catch diode (D1 Figure 1), VSW is the voltage drop of the internal R2 should be 10.0k or less to avoid bias current errors. switch, and tON(MIN) is the minimum on time of the Reference designators refer to the Block Diagram in switch, all at maximum load current. Figure 1. 2500 2250
Choosing the Switching Frequency
2000 The LT1939 switching frequency is set by resistor R5 in 1750 Figure 1. The R 1500 T/SYNC pin is internally regulated at 1V. 1250 Setting resistor R5 sets the current in the RT/SYNC pin 1000 which determines the oscillator frequency as illustrated FREQUENCY (kHz) 750 in Figure 2. 500 The switching frequency is typically set as high as pos- 250 sible to reduce overall solution size. The LT1939 employs 0 0 20 40 60 80 100 120 140 160 180 200 techniques to enhance dropout at high frequencies but RRT/SYNC (kΩ) effi ciency and maximum input voltage decrease due to 1939 G17 switching losses and minimum switch on times. The
Figure 2. Frequency vs RT/SYNC Resistance
1939f 10
OPERATION
a drive voltage higher than VIN to saturate the output NPN A power good comparator with 30mV of hysteresis trips and maintain high effi ciency. when both FB and LFB are above 90% of the 0.8V refer- ence. The PG output is an open collector NPN that is off In addition to the switching regulator, the LT1939 contains when the output is in regulation allowing a resistor to pull a NPN linear regulator with a 0.8V reference, and 13mA the PG pin to a desired voltage. The PG output is an open- current capability. The 0.8 reference will track the SS pin collector NPN that is on when the output is in regulation in the same manner as the switching regulator. The linear providing either drive for an output disconnect transistor output can also be confi gured to drive an external NPN to or inverted power good logic. provide a linear regulator with higher current capability.
APPLICATIONS INFORMATION Choosing the Output Voltage
maximum recommended frequency can be approximated by the equation: The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- 1 Frequency (Hz)= VOUT1 + VD • tors according to: V t IN VSW + VD ON(MIN) V R1= R2 OUT1 – 1 0.8V where VD is the forward voltage drop of the catch diode (D1 Figure 1), VSW is the voltage drop of the internal R2 should be 10.0k or less to avoid bias current errors. switch, and tON(MIN) is the minimum on time of the Reference designators refer to the Block Diagram in switch, all at maximum load current. Figure 1. 2500 2250
Choosing the Switching Frequency
2000 The LT1939 switching frequency is set by resistor R5 in 1750 Figure 1. The R 1500 T/SYNC pin is internally regulated at 1V. 1250 Setting resistor R5 sets the current in the RT/SYNC pin 1000 which determines the oscillator frequency as illustrated FREQUENCY (kHz) 750 in Figure 2. 500 The switching frequency is typically set as high as pos- 250 sible to reduce overall solution size. The LT1939 employs 0 0 20 40 60 80 100 120 140 160 180 200 techniques to enhance dropout at high frequencies but RRT/SYNC (kΩ) effi ciency and maximum input voltage decrease due to 1939 G17 switching losses and minimum switch on times. The
Figure 2. Frequency vs RT/SYNC Resistance
1939f 10
