Datasheet LT3435 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | High Voltage 3A, 500kHz Step-Down Switching Regulator with 100µA Quiescent Current |
| Pages / Page | 24 / 9 — BLOCK DIAGRA. APPLICATIO S I FOR ATIO. FEEDBACK PIN FUNCTIONS. Table 1. … |
| File Format / Size | PDF / 327 Kb |
| Document Language | English |
BLOCK DIAGRA. APPLICATIO S I FOR ATIO. FEEDBACK PIN FUNCTIONS. Table 1. OUTPUT. VOLTAGE. NEAREST (1%). ERROR. (V). , 1%). (%)
Model Line for this Datasheet
Text Version of Document
LT3435
W BLOCK DIAGRA
power from the VIN pin, but if the BIAS pin is connected to To further optimize efficiency, the LT3435 automatically an external voltage higher than 3V bias power will be switches to Burst Mode operation in light load situations. drawn from the external source (typically the regulated In Burst Mode operation, all circuitry associated with output voltage). This improves efficiency. controlling the output switch is shut down reducing the input supply current to 45µA. High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher The LT3435 contains a power good flag with a program- than the input voltage, allowing switch to be saturated. mable threshold and delay time. A logic-level low on the This boosted voltage is generated with an external capaci- SHDN pin disables the IC and reduces input suppy current tor and diode. to less than 1µA.
U U W U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS Table 1
The feedback (FB) pin on the LT3435 is used to set output
OUTPUT R1 OUTPUT VOLTAGE R2 NEAREST (1%) ERROR
voltage and provide several overload protection features.
(V) (k
Ω
, 1%) (k
Ω
) (%)
The first part of this section deals with selecting resistors 2.5 100 100 0 to set output voltage and the remaining part talks about 3 100 140 0 frequency foldback and soft-start features. Please read 3.3 100 165 0.38 both parts before committing to a final design. 5 100 301 0.25 Referring to Figure 2, the output voltage is determined by 6 100 383 0.63 a voltage divider from VOUT to ground which generates 8 100 536 – 0.63 1.25V at the FB pin. Since the output divider is a load on the 10 100 698 – 0.25 output care must be taken when choosing the resistor 12 100 866 0.63 divider values. For light load applications the resistor values should be as large as possible to achieve peak efficiency in Burst Mode operation. Extremely large values for resistor R1 will cause an output voltage error due to the VOUT LT3435 SW 2 50nA FB pin input current. The suggested value for the C1 C output divider resistor (see Figure 2) from FB to ground SS SOFT-START 9 (R2) is 100k or less. A formula for R1 is shown below. A table of standard 1% values is shown in Table 1 for 500kHz FOLDBACK R1 common output voltages. OSCILLATOR DETECT V – 1.25 FB R = R OUT 1 2 • – 12 1.25 + R2 • 5 n 0 A ERROR AMP R2 +
More Than Just Voltage Feedback
1.25V The FB pin is used for more than just output voltage VC 11 sensing. It also reduces switching frequency and con- 3435 F02 trols the soft-start voltage ramp rate when output voltage is below the regulated level (see the Frequency Foldback
Figure 2. Feedback Network
3435fa 9
W BLOCK DIAGRA
power from the VIN pin, but if the BIAS pin is connected to To further optimize efficiency, the LT3435 automatically an external voltage higher than 3V bias power will be switches to Burst Mode operation in light load situations. drawn from the external source (typically the regulated In Burst Mode operation, all circuitry associated with output voltage). This improves efficiency. controlling the output switch is shut down reducing the input supply current to 45µA. High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher The LT3435 contains a power good flag with a program- than the input voltage, allowing switch to be saturated. mable threshold and delay time. A logic-level low on the This boosted voltage is generated with an external capaci- SHDN pin disables the IC and reduces input suppy current tor and diode. to less than 1µA.
U U W U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS Table 1
The feedback (FB) pin on the LT3435 is used to set output
OUTPUT R1 OUTPUT VOLTAGE R2 NEAREST (1%) ERROR
voltage and provide several overload protection features.
(V) (k
Ω
, 1%) (k
Ω
) (%)
The first part of this section deals with selecting resistors 2.5 100 100 0 to set output voltage and the remaining part talks about 3 100 140 0 frequency foldback and soft-start features. Please read 3.3 100 165 0.38 both parts before committing to a final design. 5 100 301 0.25 Referring to Figure 2, the output voltage is determined by 6 100 383 0.63 a voltage divider from VOUT to ground which generates 8 100 536 – 0.63 1.25V at the FB pin. Since the output divider is a load on the 10 100 698 – 0.25 output care must be taken when choosing the resistor 12 100 866 0.63 divider values. For light load applications the resistor values should be as large as possible to achieve peak efficiency in Burst Mode operation. Extremely large values for resistor R1 will cause an output voltage error due to the VOUT LT3435 SW 2 50nA FB pin input current. The suggested value for the C1 C output divider resistor (see Figure 2) from FB to ground SS SOFT-START 9 (R2) is 100k or less. A formula for R1 is shown below. A table of standard 1% values is shown in Table 1 for 500kHz FOLDBACK R1 common output voltages. OSCILLATOR DETECT V – 1.25 FB R = R OUT 1 2 • – 12 1.25 + R2 • 5 n 0 A ERROR AMP R2 +
More Than Just Voltage Feedback
1.25V The FB pin is used for more than just output voltage VC 11 sensing. It also reduces switching frequency and con- 3435 F02 trols the soft-start voltage ramp rate when output voltage is below the regulated level (see the Frequency Foldback
Figure 2. Feedback Network
3435fa 9
