Datasheet LTC3408 (Analog Devices) - 6
| Manufacturer | Analog Devices |
| Description | 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor |
| Pages / Page | 12 / 6 — FU CTIO AL DIAGRA. OPERATIO (Refer to Functional Diagram). Controlling … |
| File Format / Size | PDF / 165 Kb |
| Document Language | English |
FU CTIO AL DIAGRA. OPERATIO (Refer to Functional Diagram). Controlling the Output Voltage. Figure 1. Typical Application
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Text Version of Document
LTC3408
U U W FU CTIO AL DIAGRA
SLOPE COMP OSC OSC FREQ 2 VIN ÷2 – REF + – 6 SLEEP + 5Ω 7 VIN V 0.85V + – + OUT ICOMP FB – EA BURST 8 V 360k OUT + S Q 1 IBCMP– R Q 180k SWITCHING RS LATCH LOGIC P-CHANNEL AND ANTI- BLANKING SHOOT- VIN CIRCUIT THRU 4 SW – BCMP 1.2V + + RUN 9 5 IRCMP– 3 GND 3408 BD
U OPERATIO (Refer to Functional Diagram)
V 4.7µH* IN VOUT output voltage can respond quickly to the external refer- 2.7V VIN SW 3× VREF TO 5V C † IN C 600mA LTC3408 OUT** ence voltage by sourcing or sinking current as needed. 10µF 4.7µF CER RUN VOUT CER REF REF
Controlling the Output Voltage
GND *MURATA LQH32CN4R7M11 **TAIYO YUDEN JMK212BJ475MG † The output voltage can be dynamically programmed from 3403 F01 TAIYO YUDEN JMK212BJ106MN 0.3V to 3.5V using the REF input. Because the gain to VOUT
Figure 1. Typical Application
from REF is internally set to 3, the corresponding input range at REF is 0.1V to 1.167V. VOUT can be modulated
Main Control Loop
during operation by driving REF with an external DAC. The LTC3408 uses a constant frequency, current mode step- When REF exceeds 1.2V, a 0.08Ω internal bypass P-channel down architecture. The main (P-channel MOSFET), syn- MOSFET connects VIN to VOUT, dramatically reducing the chronous (N-channel MOSFET) and bypass (P-channel drop across the inductor and the main switch. MOSFET) switches are internal. During normal operation, the internal main switch is turned on each cycle when the
Short-Circuit Protection
oscillator sets the RS latch, and turned off when the cur- A current sense comparator monitors the current across rent comparator, ICOMP, resets the RS latch. The peak in- the bypass P-channel MOSFET with a trip current of about ductor current at which ICOMP resets the RS latch, is con- 2.5A. When this current is exceeded during a V trolled by the output of error amplifier EA. When the load OUT short to ground, the bypass P-channel MOSFET is immediately current increases, it causes a slight decrease in the feed- turned off. The propagation delay of the current sensing back voltage, FB, relative to the external reference, which comparator, I in turn, causes the EA amplifier’s output voltage to increase BCMP, detecting an overcurrent condition to turning off the bypass P-channel MOSFET is approxmately until the average inductor current matches the new load cur- 100ns. Once the bypass P-channel MOSFET is off for about rent. While the main switch is off, the synchronous switch 10µs to 20µs, it is allowed to turn back on. The initial is turned on until the beginning of the next clock cycle. current limit is then lowered to about 1.6A after the first The LTC3408 operates in forced continuous mode where current limit trip. If the short to ground persists, the cur- the inductor current is constantly cycled. In this mode, the rent comparator will trip at the lower current limit, turning 3408f 6
U U W FU CTIO AL DIAGRA
SLOPE COMP OSC OSC FREQ 2 VIN ÷2 – REF + – 6 SLEEP + 5Ω 7 VIN V 0.85V + – + OUT ICOMP FB – EA BURST 8 V 360k OUT + S Q 1 IBCMP– R Q 180k SWITCHING RS LATCH LOGIC P-CHANNEL AND ANTI- BLANKING SHOOT- VIN CIRCUIT THRU 4 SW – BCMP 1.2V + + RUN 9 5 IRCMP– 3 GND 3408 BD
U OPERATIO (Refer to Functional Diagram)
V 4.7µH* IN VOUT output voltage can respond quickly to the external refer- 2.7V VIN SW 3× VREF TO 5V C † IN C 600mA LTC3408 OUT** ence voltage by sourcing or sinking current as needed. 10µF 4.7µF CER RUN VOUT CER REF REF
Controlling the Output Voltage
GND *MURATA LQH32CN4R7M11 **TAIYO YUDEN JMK212BJ475MG † The output voltage can be dynamically programmed from 3403 F01 TAIYO YUDEN JMK212BJ106MN 0.3V to 3.5V using the REF input. Because the gain to VOUT
Figure 1. Typical Application
from REF is internally set to 3, the corresponding input range at REF is 0.1V to 1.167V. VOUT can be modulated
Main Control Loop
during operation by driving REF with an external DAC. The LTC3408 uses a constant frequency, current mode step- When REF exceeds 1.2V, a 0.08Ω internal bypass P-channel down architecture. The main (P-channel MOSFET), syn- MOSFET connects VIN to VOUT, dramatically reducing the chronous (N-channel MOSFET) and bypass (P-channel drop across the inductor and the main switch. MOSFET) switches are internal. During normal operation, the internal main switch is turned on each cycle when the
Short-Circuit Protection
oscillator sets the RS latch, and turned off when the cur- A current sense comparator monitors the current across rent comparator, ICOMP, resets the RS latch. The peak in- the bypass P-channel MOSFET with a trip current of about ductor current at which ICOMP resets the RS latch, is con- 2.5A. When this current is exceeded during a V trolled by the output of error amplifier EA. When the load OUT short to ground, the bypass P-channel MOSFET is immediately current increases, it causes a slight decrease in the feed- turned off. The propagation delay of the current sensing back voltage, FB, relative to the external reference, which comparator, I in turn, causes the EA amplifier’s output voltage to increase BCMP, detecting an overcurrent condition to turning off the bypass P-channel MOSFET is approxmately until the average inductor current matches the new load cur- 100ns. Once the bypass P-channel MOSFET is off for about rent. While the main switch is off, the synchronous switch 10µs to 20µs, it is allowed to turn back on. The initial is turned on until the beginning of the next clock cycle. current limit is then lowered to about 1.6A after the first The LTC3408 operates in forced continuous mode where current limit trip. If the short to ground persists, the cur- the inductor current is constantly cycled. In this mode, the rent comparator will trip at the lower current limit, turning 3408f 6
