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LTC3127 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3127 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LTC3127 3127f operaTion The input current limit is set by the RPROG resistor placed on the PROG pin to SGND. The resistor value can be calculated using the following formula: RPROG = 54.92 • ILIMIT (A) + 4.94 (kΩ) Where ILIMIT is the average input current limit in amps. A secondary 2.5A (typical) current limit forces switches B and D on and A and C off if tripped. This current limit is not affected by the value of RPROG. Reverse Current Limit The reverse current comparator on switch D monitors the inductor current supplied from the output. When this current exceeds 300mA (typical) switch D will be turned off for the remainder of the switching cycle. Burst Mode Operation When the MODE pin is held high the LTC3127 will func- tion in Burst Mode operation as long as the load current is typically less than 35mA. In Burst Mode operation, the LTC3127 still switches at a fixed frequency of 1.35MHz, usingthesameerroramplifiersandloopcompensationfor average input current mode control. This control method eliminates any output transient when switching between modes. In Burst Mode operation, energy is delivered to the output until the output voltage reaches the nominal regulation value. At this point, the LTC3127 transitions to sleep mode where the output switches are shut off and the LTC3127 consumes only 35μA of quiescent current from VIN. When the output voltage droops slightly, switching resumes. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Zero Current Comparator The zero current comparator monitors the inductor cur- rent to the output and shuts off the synchronous rectifier when this current reduces to approximately 30mA. This prevents the inductor current from reversing in polarity, improving efficiency at light loads. This comparator is only active in Burst Mode operation. Anti-Ringing Control The anti-ringing control connects a resistor from SW1 and SW2 to PGND to prevent high frequency ringing during discontinuous current mode operation in Burst Mode. Although the ringing of the resonant circuit formed by L and CSW (capacitance on SW pin) is low energy, it can cause EMI radiation. Shutdown Shutdown of the converter is accomplished by pulling SHDN below 0.3V and enabled by pulling SHDN above 1.2V. Note that SHDN can be driven above VIN or VOUT, as long as it is limited to less than the absolute maximum rating. Thermal Shutdown Ifthedietemperatureexceeds150°C(typical)theLTC3127 will be disabled. All power devices will be turned off and both switch nodes will be high impedance. The LTC3127 will restart (if enabled) when the die temperature drops to approximately 140°C. Thermal Regulator To help prevent the part from going into thermal shutdown when charging very large capacitive loads, the LTC3127 is equipped with a thermal regulator. If the die temperature exceeds 130°C (typical) the average current limit is lowered to help reduce the amount of power being dissipated in the package.Thecurrentlimitwillbeapproximately0Ajustbefore thermalshutdown.Thecurrentlimitwillreturntoitsfullvalue when the die temperature drops back below 130°C. Undervoltage Lockout If the input supply voltage drops below 1.7V (typical), the LTC3127 will be disabled and all power devices will be turned off. |
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