Datasheet LTC3459 (Analog Devices) - 7
| Fabricante | Analog Devices |
| Descripción | 10V Micropower Synchronous Boost Converter in ThinSOT |
| Páginas / Página | 12 / 7 — OPERATION. Operation. Peak Current Overshoot. tOFF Timer. Figure 1. … |
| Formato / tamaño de archivo | PDF / 226 Kb |
| Idioma del documento | Inglés |
OPERATION. Operation. Peak Current Overshoot. tOFF Timer. Figure 1. Inductor Current and VOUT Ripple Waveforms
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LTC3459
OPERATION Operation
converter disconnects VOUT from VIN during shutdown to avoid loading the input power source. The LTC3459 synchronous boost converter utilizes a Burst Mode control technique to achieve high effi ciency
Peak Current Overshoot
over a wide dynamic range. A 2.5% accurate comparator is used to monitor the output voltage (V The LTC3459’s peak current comparator has a delay of ap- OUT), if VOUT is above the comparator threshold, no switching occurs and proximately 100ns from the time inductor current reaches only quiescent current (10μA) is drawn from the power current limit until the internal N-channel MOSFET turns off. source. When V This delay causes the peak current to overshoot based on OUT drops below the comparator threshold, switching commences and the output capacitor is charged. the inductor value and VIN, as follows (Figure 2 is based During the on time of the switching period, inductor cur- on a 65mA initial ILIMIT). rent is ramped through an internal N-channel MOSFET to V I I n 100 s IN = + ( ) GND until a peak current (75mA) is detected. A P-channel PEAK LIMIT L MOSFET connects the inductor to VOUT during the off time delivering energy to the load. The off time is controlled by
tOFF Timer
an internal timer which is proportional to 1/(VOUT – VIN). The LTC3459’s t Anticross conduction circuitry ensures the N- and P-chan- OFF timer is designed to keep the inductor current continuous during a Burst Mode switching packet, nel switches are never on simultaneously. thereby increasing current capability at the output. A larger Only three power components and two feedback resistors inductor value will have lower peak-to-peak current ripple, are required to complete the design of the boost converter, increasing the available current to the load. This improve- an external Schottky diode is not required. The high op- ment is offset somewhat by the reduced IPEAK overshoot. erating frequency allows the use of low value, low profi le The tOFF timer is designed to maintain a relatively constant inductors and tiny external ceramic capacitors. The boost peak-to-peak current in the inductor despite VIN changes. ~50mVP-P VOUT AC RIPPLE IPEAK ~100mA t t t t OFF OFF OFF OFF N P N P N P N P N P N BURST ON I SLEEP ZERO WAIT BURST ON 3459 F01
Figure 1. Inductor Current and VOUT Ripple Waveforms
110 0.8 0.7 100 15μH 0.6 90 22μH 0.5 (mA) 80 (μs) 0.4 I PEAK 33μH t OFF 0.3 70 0.2 60 0.1 50 0 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 VIN (V) 3459 F02 VOUT – VIN (V) 3459 F03
Figure 2. Typical IPEAK Values Figure 3. tOFF Times
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OPERATION Operation
converter disconnects VOUT from VIN during shutdown to avoid loading the input power source. The LTC3459 synchronous boost converter utilizes a Burst Mode control technique to achieve high effi ciency
Peak Current Overshoot
over a wide dynamic range. A 2.5% accurate comparator is used to monitor the output voltage (V The LTC3459’s peak current comparator has a delay of ap- OUT), if VOUT is above the comparator threshold, no switching occurs and proximately 100ns from the time inductor current reaches only quiescent current (10μA) is drawn from the power current limit until the internal N-channel MOSFET turns off. source. When V This delay causes the peak current to overshoot based on OUT drops below the comparator threshold, switching commences and the output capacitor is charged. the inductor value and VIN, as follows (Figure 2 is based During the on time of the switching period, inductor cur- on a 65mA initial ILIMIT). rent is ramped through an internal N-channel MOSFET to V I I n 100 s IN = + ( ) GND until a peak current (75mA) is detected. A P-channel PEAK LIMIT L MOSFET connects the inductor to VOUT during the off time delivering energy to the load. The off time is controlled by
tOFF Timer
an internal timer which is proportional to 1/(VOUT – VIN). The LTC3459’s t Anticross conduction circuitry ensures the N- and P-chan- OFF timer is designed to keep the inductor current continuous during a Burst Mode switching packet, nel switches are never on simultaneously. thereby increasing current capability at the output. A larger Only three power components and two feedback resistors inductor value will have lower peak-to-peak current ripple, are required to complete the design of the boost converter, increasing the available current to the load. This improve- an external Schottky diode is not required. The high op- ment is offset somewhat by the reduced IPEAK overshoot. erating frequency allows the use of low value, low profi le The tOFF timer is designed to maintain a relatively constant inductors and tiny external ceramic capacitors. The boost peak-to-peak current in the inductor despite VIN changes. ~50mVP-P VOUT AC RIPPLE IPEAK ~100mA t t t t OFF OFF OFF OFF N P N P N P N P N P N BURST ON I SLEEP ZERO WAIT BURST ON 3459 F01
Figure 1. Inductor Current and VOUT Ripple Waveforms
110 0.8 0.7 100 15μH 0.6 90 22μH 0.5 (mA) 80 (μs) 0.4 I PEAK 33μH t OFF 0.3 70 0.2 60 0.1 50 0 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 VIN (V) 3459 F02 VOUT – VIN (V) 3459 F03
Figure 2. Typical IPEAK Values Figure 3. tOFF Times
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