LTC3832/LTC3832-1 UUWUAPPLICATIO S I FOR ATIO remains at a reduced voltage until the overload is re- meant to prevent damage to the power supply circuitry moved. Serious overloads generate a large overdrive at during fault conditions. The exact current level where the CC, allowing it to pull SS down quickly and preventing limiting circuit begins to take effect will vary from unit to damage to the output components. By using the RDS(ON) unit as the RDS(ON) of Q1 varies. Typically, RDS(ON) varies of Q1 to measure the output current, the current limiting as much as ±40%, and with ±33% variation on the circuit eliminates an expensive discrete sense resistor that LTC3832’s IMAX current, this can give a ±73% variation on would otherwise be required. This helps minimize the the current limit threshold. number of components in the high current path. The RDS(ON) is high if the VGS applied to the MOSFET is The current limit threshold can be set by connecting an low. This occurs during power up when PVCC1 is ramping external resistor RIMAX from the IMAX pin to the main VIN up. To prevent the high RDS(ON) from activating the current supply at the drain of Q1. The value of RIMAX is determined limit, the LTC3832 will disable the current limit circuit if by: PVCC1 is less than 2.5V above VCC. To ensure proper operation of the current limit circuit, PV R CC1 must be at IMAX = (ILMAX)(RDS(ON)Q1)/IIMAX least 2.5V above VCC when G1 is high. PVCC1 can go low where: when G1 is low, allowing the use of the external charge I pump to power PV LMAX = ILOAD + (IRIPPLE/2) CC1. V I IN LOAD = Maximum load current I LTC3832 + RIPPLE = Inductor ripple current RIMAX 0.1µF CIN (V – V + IN OUT)(VOUT ) 12 = ( I 12µA MAX fOSC)(LO)(VIN) CC G1 Q1 IFB 1k LO – 13 VOUT f + OSC = LTC3832 oscillator frequency = 300kHz G2 Q2 COUT LO = Inductor value 3832 F04 RDS(ON)Q1 = On-resistance of Q1 at ILMAX Figure 4. Current Limit Setting IIMAX= Internal 12µA sink current at IMAX Oscillator Frequency The RDS(ON) of Q1 usually increases with temperature. To keep the current limit threshold constant, the internal The LTC3832 includes an onboard current controlled 12µA sink current at I oscillator that typically free-runs at 300kHz. The oscillator MAX is designed with a positive temperature coefficient to provide first order correction frequency can be adjusted by forcing current into or out of for the temperature coefficient of R the FREQSET pin. With the pin floating, the oscillator runs DS(ON)Q1. at about 300kHz. Every additional 1µA of current into/out In order for the current limit circuit to operate properly and of the FREQSET pin decreases/increases the frequency by to obtain a reasonably accurate current limit threshold, the 10kHz. The pin is internally servoed to 1.265V. The IIMAX and IFB pins must be Kelvin sensed at Q1’s drain and frequency can be estimated as: source pins. In addition, connect a 0.1µF decoupling capacitor across RIMAX to filter switching noise. Other- 1 265 . V – V kHz EXT 10 wise, noise spikes or ringing at Q1’s source can cause the f = kHz 300 + • R 1 A µ actual maximum current to be greater than the desired FSET current limit set point. Due to switching noise and varia- where RFSET is a frequency programming resistor con- tion of RDS(ON), the actual current limit trip point is not nected between FREQSET and the external voltage source highly accurate. The current limiting circuitry is primarily VEXT. sn3832 3832fs 10
