Datasheet LTC3830, LTC3830-1 (Analog Devices) - 9

LTC3830/LTC3830-1
U U W U APPLICATIO S I FOR ATIO
Similarly, the MAX comparator forces the output to 0% the voltage at IFB to the voltage at the IMAX pin. As the peak duty cycle if the feedback signal is greater than 40mV current rises, the measured voltage across Q1 increases above the internal reference. To prevent these two com- due to the drop across the RDS(ON) of Q1. When the voltage parators from triggering due to noise, the MIN and MAX at IFB drops below IMAX, indicating that Q1’s drain current comparators’ response times are deliberately delayed by has exceeded the maximum level, CC starts to pull current two to three microseconds. These two comparators help out of CSS, cutting the duty cycle and controlling the output prevent extreme output perturbations with fast output current level. The CC comparator pulls current out of the load current transients, while allowing the main feedback SS pin in proportion to the voltage difference between IFB loop to be optimally compensated for stability. and IMAX. Under minor overload conditions, the SS pin falls gradually, creating a time delay before current limit
Thermal Shutdown
takes effect. Very short, mild overloads may not affect the The LTC3830/LTC3830-1 have a thermal protection cir- output voltage at all. More significant overload conditions cuit that disables both gate drivers if activated. If the chip allow the SS pin to reach a steady state, and the output junction temperature reaches 150°C, both G1 and G2 are remains at a reduced voltage until the overload is re- pulled low. G1 and G2 remain low until the junction moved. Serious overloads generate a large overdrive at temperature drops below 125°C, after which, the chip CC, allowing it to pull SS down quickly and preventing resumes normal operation. damage to the output components. By using the RDS(ON) of Q1 to measure the output current, the current limiting
Soft-Start and Current Limit
circuit eliminates an expensive discrete sense resistor that would otherwise be required. This helps minimize the The 16-lead LTC3830 devices include a soft-start circuit number of components in the high current path. that is used for start-up and current limit operation. The LTC3830-1 only has the soft-start function; the current The current limit threshold can be set by connecting an limit function is disabled. The 8-lead LTC3830 has both the external resistor RIMAX from the IMAX pin to the main VIN soft-start and current limit function disabled. The SS pin supply at the drain of Q1. The value of RIMAX is determined requires an external capacitor, C by: SS, to GND with the value determined by the required soft-start time. An internal RIMAX = (ILMAX)(RDS(ON)Q1)/IIMAX 12µA current source is included to charge CSS. During power-up, the COMP pin is clamped to a diode drop (B-E where: junction of QSS in the Block Diagram) above the voltage at ILMAX = ILOAD + (IRIPPLE/2) the SS pin. This prevents the error amplifier from forcing I the loop to maximum duty cycle. The LTC3830/LTC3830-1 LOAD = Maximum load current operate at low duty cycle as the SS pin rises above 0.6V IRIPPLE = Inductor ripple current (VCOMP ≈ 1.2V). As SS continues to rise, QSS turns off and (V – V IN OUT)(VOUT ) the error amplifier takes over to regulate the output. The = f L V MIN comparator is disabled during soft-start to prevent it ( OSC)( O)( IN) from overriding the soft-start function. fOSC = LTC3830 oscillator frequency = 200kHz The 16-lead LTC3830 devices include yet another feed- LO = Inductor value back loop to control operation in current limit. Just before R every falling edge of G1, the current comparator, CC, DS(ON)Q1 = On-resistance of Q1 at ILMAX samples and holds the voltage drop measured across the IIMAX= Internal 12µA sink current at IMAX external upper MOSFET, Q1, at the IFB pin. CC compares 3830fa 9