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LTC3832EGN Datasheet(PDF) 10 Page - Linear Technology |
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LTC3832EGN Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  10 LTC3832/LTC3832-1 sn3832 3832fs remains at a reduced voltage until the overload is re- moved. Serious overloads generate a large overdrive at CC, allowing it to pull SS down quickly and preventing damage to the output components. By using the RDS(ON) of Q1 to measure the output current, the current limiting circuit eliminates an expensive discrete sense resistor that would otherwise be required. This helps minimize the number of components in the high current path. The current limit threshold can be set by connecting an external resistor RIMAX from the IMAX pin to the main VIN supply at the drain of Q1. The value of RIMAX is determined by: RIMAX = (ILMAX)(RDS(ON)Q1)/IIMAX where: ILMAX = ILOAD + (IRIPPLE/2) ILOAD = Maximum load current IRIPPLE = Inductor ripple current = ()( ) ()()( ) VV V fL V IN OUT OUT OSC O IN – fOSC = LTC3832 oscillator frequency = 300kHz LO = Inductor value RDS(ON)Q1 = On-resistance of Q1 at ILMAX IIMAX= Internal 12µA sink current at IMAX The RDS(ON) of Q1 usually increases with temperature. To keep the current limit threshold constant, the internal 12 µA sink current at IMAX is designed with a positive temperature coefficient to provide first order correction for the temperature coefficient of RDS(ON)Q1. In order for the current limit circuit to operate properly and to obtain a reasonably accurate current limit threshold, the IIMAX and IFB pins must be Kelvin sensed at Q1’s drain and source pins. In addition, connect a 0.1 µF decoupling capacitor across RIMAX to filter switching noise. Other- wise, noise spikes or ringing at Q1’s source can cause the actual maximum current to be greater than the desired current limit set point. Due to switching noise and varia- tion of RDS(ON), the actual current limit trip point is not highly accurate. The current limiting circuitry is primarily meant to prevent damage to the power supply circuitry during fault conditions. The exact current level where the limiting circuit begins to take effect will vary from unit to unit as the RDS(ON) of Q1 varies. Typically, RDS(ON) varies as much as ±40%, and with ±33% variation on the LTC3832’s IMAX current, this can give a ±73% variation on the current limit threshold. The RDS(ON) is high if the VGS applied to the MOSFET is low. This occurs during power up when PVCC1 is ramping up. To prevent the high RDS(ON) from activating the current limit, the LTC3832 will disable the current limit circuit if PVCC1 is less than 2.5V above VCC. To ensure proper operation of the current limit circuit, PVCC1 must be at least 2.5V above VCC when G1 is high. PVCC1 can go low when G1 is low, allowing the use of the external charge pump to power PVCC1. APPLICATIO S I FOR ATIO Figure 4. Current Limit Setting + + 12 13 LTC3832 CC 12 µA 0.1 µF Q2 COUT 3832 F04 CIN VIN VOUT G2 IMAX RIMAX IFB 1k + – Q1 LO G1 Oscillator Frequency The LTC3832 includes an onboard current controlled oscillator that typically free-runs at 300kHz. The oscillator frequency can be adjusted by forcing current into or out of the FREQSET pin. With the pin floating, the oscillator runs at about 300kHz. Every additional 1 µA of current into/out of the FREQSET pin decreases/increases the frequency by 10kHz. The pin is internally servoed to 1.265V. The frequency can be estimated as: f kHz VV R kHz A EXT FSET =+ µ 300 1 265 10 1 .– • where RFSET is a frequency programming resistor con- nected between FREQSET and the external voltage source VEXT. |
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