Datasheet LTC3499, LTC3499B (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | 750mA Synchronous Step-Up DC/DC Converters with Reverse-Battery Protection |
| Pages / Page | 16 / 8 — OPERATION. Error Amplifier. Current Sensing. LOW NOISE FIXED FREQUENCY … |
| File Format / Size | PDF / 252 Kb |
| Document Language | English |
OPERATION. Error Amplifier. Current Sensing. LOW NOISE FIXED FREQUENCY OPERATION. Shutdown. Antiringing Control. Soft-Start
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LTC3499/LTC3499B
OPERATION
The LTC3499/LTC3499B provide high efficiency, low noise In the event of an external shutdown or thermal shutdown power for boost applications with output voltages up to (TSD), CSS is discharged through a nominal 5kΩ imped- 6V. Operation can be best understood by referring to ance to GND. Once the condition is removed and SS is the Functional Block Diagram in Figure 1. The synchro- discharged near ground, a soft-start will automatically nous boost converters are housed in either an 8-lead be re-initiated. (3mm × 3mm) DFN or MSOP package and operates at a fixed 1.2MHz. With a 1.6V typical minimum VIN voltage
Error Amplifier
these devices are well suited for applications using two A transconductance amplifier generates an error voltage or three alkaline or nickel-metal hydride (NiMH) cells or from the difference between the positive input internally one Lithium-Ion (Li-Ion) cell. The LTC3499/LTC3499B connected to the 1.22V reference and the negative input have integrated circuitry which protects the battery, IC, connected to FB. A simple compensation network is placed and circuitry powered by the device in the event that the from VC to ground. Internal clamps limit the minimum and input batteries are connected backwards (reverse battery maximum error amplifier output voltage for improved large protection). The true output disconnect feature eliminates signal transient response. A voltage divider from V inrush current and allows V OUT to OUT to be zero volts during GND programs the output voltage via FB from 2V to 6V shutdown. The current mode architecture simplifies loop and is defined by the following equation: compensation with excellent load transient response. The low RDS(ON), low gate charge synchronous switches R1 V eliminate the need for an external Schottky diode recti- OUT = 1.22 • 1+ R2 fier, and provide efficient high frequency pulse width modulation (PWM). Burst Mode quiescent current to the
Current Sensing
LTC3499 is only 20µA from VIN, maximizing battery life. The LTC3499B does not have Burst Mode operation and Lossless current sensing converts the peak current signal the device continues switching at constant frequency. This into a voltage which is summed with the internal slope results in the absence of low frequency output ripple at compensation. This summed signal is compared to the the expense of light load efficiency. error amplifier output to provide a peak current control command for the PWM. Peak switch current is limited
LOW NOISE FIXED FREQUENCY OPERATION
to 750mA minimum.
Shutdown Antiringing Control
The LTC3499/LTC3499B are shut down by pulling SHDN The antiringing control connects a resistor across the below 0.2V, and activated by pulling the pin above 1.2V. inductor to damp the ringing on SW in discontinuous SHDN can be driven above V conduction mode. The LC resonant ringing (L = inductor, IN or VOUT as long as it is limited to less than the absolute maximum rating. CSW = capacitance on SW) is low energy, but can cause EMI radiation if antiringing control is not present.
Soft-Start Zero Current Comparator
The soft-start time is programmed with an external capaci- tor to ground on SS. An internal current source charges The zero current comparator monitors the inductor current the capacitor, C to the output and shuts off the synchronous rectifier once SS, with a nominal 3µA. The voltage on SS is used to clamp the voltage on VC. The soft-start time this current reduces to approximately 40mA, preventing is given by negative inductor current. t(msec) = CSS (µF) • 200 3499fc 8
OPERATION
The LTC3499/LTC3499B provide high efficiency, low noise In the event of an external shutdown or thermal shutdown power for boost applications with output voltages up to (TSD), CSS is discharged through a nominal 5kΩ imped- 6V. Operation can be best understood by referring to ance to GND. Once the condition is removed and SS is the Functional Block Diagram in Figure 1. The synchro- discharged near ground, a soft-start will automatically nous boost converters are housed in either an 8-lead be re-initiated. (3mm × 3mm) DFN or MSOP package and operates at a fixed 1.2MHz. With a 1.6V typical minimum VIN voltage
Error Amplifier
these devices are well suited for applications using two A transconductance amplifier generates an error voltage or three alkaline or nickel-metal hydride (NiMH) cells or from the difference between the positive input internally one Lithium-Ion (Li-Ion) cell. The LTC3499/LTC3499B connected to the 1.22V reference and the negative input have integrated circuitry which protects the battery, IC, connected to FB. A simple compensation network is placed and circuitry powered by the device in the event that the from VC to ground. Internal clamps limit the minimum and input batteries are connected backwards (reverse battery maximum error amplifier output voltage for improved large protection). The true output disconnect feature eliminates signal transient response. A voltage divider from V inrush current and allows V OUT to OUT to be zero volts during GND programs the output voltage via FB from 2V to 6V shutdown. The current mode architecture simplifies loop and is defined by the following equation: compensation with excellent load transient response. The low RDS(ON), low gate charge synchronous switches R1 V eliminate the need for an external Schottky diode recti- OUT = 1.22 • 1+ R2 fier, and provide efficient high frequency pulse width modulation (PWM). Burst Mode quiescent current to the
Current Sensing
LTC3499 is only 20µA from VIN, maximizing battery life. The LTC3499B does not have Burst Mode operation and Lossless current sensing converts the peak current signal the device continues switching at constant frequency. This into a voltage which is summed with the internal slope results in the absence of low frequency output ripple at compensation. This summed signal is compared to the the expense of light load efficiency. error amplifier output to provide a peak current control command for the PWM. Peak switch current is limited
LOW NOISE FIXED FREQUENCY OPERATION
to 750mA minimum.
Shutdown Antiringing Control
The LTC3499/LTC3499B are shut down by pulling SHDN The antiringing control connects a resistor across the below 0.2V, and activated by pulling the pin above 1.2V. inductor to damp the ringing on SW in discontinuous SHDN can be driven above V conduction mode. The LC resonant ringing (L = inductor, IN or VOUT as long as it is limited to less than the absolute maximum rating. CSW = capacitance on SW) is low energy, but can cause EMI radiation if antiringing control is not present.
Soft-Start Zero Current Comparator
The soft-start time is programmed with an external capaci- tor to ground on SS. An internal current source charges The zero current comparator monitors the inductor current the capacitor, C to the output and shuts off the synchronous rectifier once SS, with a nominal 3µA. The voltage on SS is used to clamp the voltage on VC. The soft-start time this current reduces to approximately 40mA, preventing is given by negative inductor current. t(msec) = CSS (µF) • 200 3499fc 8
