Datasheet LT1300 - 6
| Description | Micropower High Efficiency 3.3/5V Step-Up DC/DC Converter |
| Pages / Page | 8 / 6 — APPLICATIONS INFORMATION. Output Voltage Selection. Figure 4. Addition of … |
| File Format / Size | PDF / 230 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Output Voltage Selection. Figure 4. Addition of R1 and C3 Limit Input Current at Startup. Shutdown
Text Version of Document
LT1300
U U W U APPLICATIONS INFORMATION Output Voltage Selection
L1 10µH D1 The LT1300 can be selected to 3.3V or 5V under logic control or fixed at either by tying SELECT to ground or VIN V SW IN respectively. It is permissible to tie SELECT to a voltage 5V/3.3V SELECT SENSE OUTPUT + higher than V C1 IN as long as it does not exceed 10V. 100µF LT1300 Efficiency in 3.3V mode will be slightly less that in 5V mode + due to the fact that the diode drop is a greater percentage C2 SHDN ILIM 100µF of 3.3V than 5V. Since the bipolar switch in the LT1300 PGND GND R1 C3 gets its base drive from V 1M 0.1µF IN, no reduction in switch efficiency occurs when in 3.3V mode. When VIN exceeds the programmed output voltage the output will follow the input. This is characteristic of the simple step-up or
Figure 4. Addition of R1 and C3 Limit Input Current at Startup
“boost” converter topology. A circuit example that pro- vides a regulated output with an input voltage above or below the output (called a buck-boost or SEPIC) is shown VOUT in the Typical Applications section. 2VDIV
Shutdown
IBATTERY The converter can be turned off by pulling SHDN (pin 3) 500mA/DIV high. Quiescent current drops to 10µA in this condition. Bias current of 3µA to 5µA flows into the pin (at 2.5V input). VSHDN 10V/DIV It is recommended that SHDN not be left floating. Tie the 500µs/DIV pin to ground if the feature is not used. REP RATE = 1Hz LT1300 F5
I Figure 5. Startup Waveforms using Soft-Start Circuitry LIM Function ILOAD = 100mA, VOUT = 5V
The LT1300’s current limit (ILIM) pin can be used for soft start. Upon start-up, switching regulators require maxi- mum current from the supply. The high currents flowing 1100 1.6V ≤ VIN ≤ 5V can create IR drops along supply and ground lines and 1000 are especially demanding on alkaline batteries. By in- 900 stalling an R1 and C3 as shown in Figure 4, the switch 800 current in the LT1300 is limited to 400mA until the 15µA 700 flowing out of the ILIM pin charges up the 0.1µF capaci- tor. Input current is held to under 500mA while the 600 output voltage ramps up to 5V as shown in Figure 5. The SWITCH CURRENT (mA) 500 1Meg resistor provides a discharge path for the capacitor 400 without appreciably decreasing peak switch current. When 300 the full capability of the LT1300 is not required, peak 100 1k 10k 100k 1M RLIM (Ω) current can be reduced by changing the value of R3 as LT1300 F1B shown in Figure 6. With R3 = 0, switch current is limited
Figure 6. Peak Switch Current vs. RLIM
to approximately 400mA. 6
U U W U APPLICATIONS INFORMATION Output Voltage Selection
L1 10µH D1 The LT1300 can be selected to 3.3V or 5V under logic control or fixed at either by tying SELECT to ground or VIN V SW IN respectively. It is permissible to tie SELECT to a voltage 5V/3.3V SELECT SENSE OUTPUT + higher than V C1 IN as long as it does not exceed 10V. 100µF LT1300 Efficiency in 3.3V mode will be slightly less that in 5V mode + due to the fact that the diode drop is a greater percentage C2 SHDN ILIM 100µF of 3.3V than 5V. Since the bipolar switch in the LT1300 PGND GND R1 C3 gets its base drive from V 1M 0.1µF IN, no reduction in switch efficiency occurs when in 3.3V mode. When VIN exceeds the programmed output voltage the output will follow the input. This is characteristic of the simple step-up or
Figure 4. Addition of R1 and C3 Limit Input Current at Startup
“boost” converter topology. A circuit example that pro- vides a regulated output with an input voltage above or below the output (called a buck-boost or SEPIC) is shown VOUT in the Typical Applications section. 2VDIV
Shutdown
IBATTERY The converter can be turned off by pulling SHDN (pin 3) 500mA/DIV high. Quiescent current drops to 10µA in this condition. Bias current of 3µA to 5µA flows into the pin (at 2.5V input). VSHDN 10V/DIV It is recommended that SHDN not be left floating. Tie the 500µs/DIV pin to ground if the feature is not used. REP RATE = 1Hz LT1300 F5
I Figure 5. Startup Waveforms using Soft-Start Circuitry LIM Function ILOAD = 100mA, VOUT = 5V
The LT1300’s current limit (ILIM) pin can be used for soft start. Upon start-up, switching regulators require maxi- mum current from the supply. The high currents flowing 1100 1.6V ≤ VIN ≤ 5V can create IR drops along supply and ground lines and 1000 are especially demanding on alkaline batteries. By in- 900 stalling an R1 and C3 as shown in Figure 4, the switch 800 current in the LT1300 is limited to 400mA until the 15µA 700 flowing out of the ILIM pin charges up the 0.1µF capaci- tor. Input current is held to under 500mA while the 600 output voltage ramps up to 5V as shown in Figure 5. The SWITCH CURRENT (mA) 500 1Meg resistor provides a discharge path for the capacitor 400 without appreciably decreasing peak switch current. When 300 the full capability of the LT1300 is not required, peak 100 1k 10k 100k 1M RLIM (Ω) current can be reduced by changing the value of R3 as LT1300 F1B shown in Figure 6. With R3 = 0, switch current is limited
Figure 6. Peak Switch Current vs. RLIM
to approximately 400mA. 6
