Datasheet LTC3405A-1.5, LTC3405A-1.8 (Analog Devices) - 6
| Fabricante | Analog Devices |
| Descripción | 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT |
| Páginas / Página | 16 / 6 — FU CTIO AL DIAGRA. OPERATIO (Refer to Functional Diagram). Main Control … |
| Formato / tamaño de archivo | PDF / 209 Kb |
| Idioma del documento | Inglés |
FU CTIO AL DIAGRA. OPERATIO (Refer to Functional Diagram). Main Control Loop. Burst Mode Operation
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LTC3405A-1.5/LTC3405A-1.8
U U W FU CTIO AL DIAGRA
MODE 6 SLOPE COMP 0.65V OSC OSC FREQ 4 VIN V SHIFT – OUT 5 + – EN SLEEP R1 + LTC3405A-1.5 1.2V 5Ω 0.4V + – + R1 = 110k I – EA COMP R2 = 440k BURST LTC3405A-1.8 R2 VFB S Q R1 = 180k R2 = 360k R Q SWITCHING V RS LATCH IN LOGIC AND ANTI- SHOOT- RUN BLANKING CIRCUIT THRU 3 SW 1 1.2V REF – OV OVDET 1.294V + + SHUTDOWN IRCMP– 2 GND 3405A1518 BD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
Comparator OVDET guards against transient overshoots > 7.8% by turning the main switch off and keeping it off The LTC3405A Fixed output voltage series parts use a until the fault is removed. constant frequency, current mode step-down architec- ture. Their main (P-channel MOSFET) and synchronous
Burst Mode Operation
(N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on The LTC3405A series parts are capable of Burst Mode each cycle when the oscillator sets the RS latch, and turned operation in which the internal power MOSFETs operate off when the current comparator, I intermittently based on load demand. To enable Burst COMP, resets the RS latch. The peak inductor current at which I Mode operation, simply connect the MODE pin to GND. To COMP resets the RS latch, is controlled by the output of error amplifier EA. disable Burst Mode operation and enable PWM pulse When the load current increases, the output voltage de- skipping mode, connect the MODE pin to VIN or drive it creases which causes a slight decrease in V with a logic high (V FB relative to MODE > 1.5V). In this mode, the the 1.2V reference, which in turn, causes the EA amplifier’s efficiency is lower at light loads, but becomes comparable output voltage to increase until the average inductor to Burst Mode operation when the output load exceeds current matches the new load current. While the top 25mA. The advantage of pulse skipping mode is lower MOSFET is off, the bottom MOSFET is turned on until output ripple and less interference to audio circuitry. either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. 3405a1518fa 6
U U W FU CTIO AL DIAGRA
MODE 6 SLOPE COMP 0.65V OSC OSC FREQ 4 VIN V SHIFT – OUT 5 + – EN SLEEP R1 + LTC3405A-1.5 1.2V 5Ω 0.4V + – + R1 = 110k I – EA COMP R2 = 440k BURST LTC3405A-1.8 R2 VFB S Q R1 = 180k R2 = 360k R Q SWITCHING V RS LATCH IN LOGIC AND ANTI- SHOOT- RUN BLANKING CIRCUIT THRU 3 SW 1 1.2V REF – OV OVDET 1.294V + + SHUTDOWN IRCMP– 2 GND 3405A1518 BD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
Comparator OVDET guards against transient overshoots > 7.8% by turning the main switch off and keeping it off The LTC3405A Fixed output voltage series parts use a until the fault is removed. constant frequency, current mode step-down architec- ture. Their main (P-channel MOSFET) and synchronous
Burst Mode Operation
(N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on The LTC3405A series parts are capable of Burst Mode each cycle when the oscillator sets the RS latch, and turned operation in which the internal power MOSFETs operate off when the current comparator, I intermittently based on load demand. To enable Burst COMP, resets the RS latch. The peak inductor current at which I Mode operation, simply connect the MODE pin to GND. To COMP resets the RS latch, is controlled by the output of error amplifier EA. disable Burst Mode operation and enable PWM pulse When the load current increases, the output voltage de- skipping mode, connect the MODE pin to VIN or drive it creases which causes a slight decrease in V with a logic high (V FB relative to MODE > 1.5V). In this mode, the the 1.2V reference, which in turn, causes the EA amplifier’s efficiency is lower at light loads, but becomes comparable output voltage to increase until the average inductor to Burst Mode operation when the output load exceeds current matches the new load current. While the top 25mA. The advantage of pulse skipping mode is lower MOSFET is off, the bottom MOSFET is turned on until output ripple and less interference to audio circuitry. either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. 3405a1518fa 6