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LTC3422 6 3422fa PI FU CTIO S SW (Pin 1): Switch Pin for the Inductor Connection. Minimize trace length between SW and inductor. For discontinuous inductor current, a controlled impedance is internally connected from SW to VIN to eliminate high frequency ringing, reducing EMI radiation. VIN (Pin 2): Input Supply Voltage. Connect VIN to the input supply and decouple with a 4.7 µF or larger ceramic capacitor as close to VIN as possible. BURST (Pin 3): Burst Mode Threshold Adjust. Automatic Burst Mode Operation: A resistor/capacitor combination from BURST to ground programs the average load current at which automatic Burst Mode operation is exited, ac- cording to the formula: R I B EXITBURST = 12 where RB is in kΩ and IEXITBURST is in amps C CV B OUT OUT ≥ • , 64 000 where CB(MIN) and COUT are in µF. Please refer to the Burst Mode Output Current Threshold vs RBURST Typical Performance Chacteristic curves. Note that during Burst Mode operation the peak inductor current will be approximately 600mA and return to zero on each cycle. In Burst Mode operation the frequency is variable, providing a significant efficiency improvement at light loads. The LTC3422 only allows Burst Mode operation to be entered once VOUT exceeds approximately 2.2V. Manually Implementing Burst Mode Operation: Ground BURST to force Burst Mode operation or connect it to VOUT to force fixed frequency PWM mode. Note that BURST must not be pulled higher than VOUT. SS (Pin 4): Soft-Start. Connect a capacitor from SS to ground to set the soft-start time according to the formula: t(ms) = CSS(µF) • 320 The nominal soft-start charging current is 2.4 µA. The active range of SS is from 0.8V to 1.6V. SHDN (Pin 5): Shutdown Input. Less than 250mV on SHDN shuts down the LTC3422. Placing 1V or more on SHDN enables the LTC3422. Once VOUT exceeds 2.2V, hysteresis is applied to this pin (500nA exits the pin) allowing it to operate at a logic high while the battery can drop to 500mV. FB (Pin 6): Feedback Input to Error Amplifier. Connect the VOUT to ground resistor divider tap here. The output voltage can be adjusted from 2.25V to 5.25V according to the formula: V RR R OUT = + 1 216 12 2 .• VC (Pin 7): Error Amp Output. A frequency compensation network is connected from VC to ground to compensate the loop. See the section “Closing the Feedback Loop” for guide- lines. RT(Pin8):FrequencyAdjustInput.Connectaresistortoground to program the oscillator frequency according to the formula: f R OSC T = 28 where fOSC is in MHz and RT is in kΩ. SYNC (Pin 9): Oscillator Synchronization Input. A clock pulse width of 100ns to 2 µs is required to synchronize the internal oscillator. If not used, SYNC should be grounded. VOUT (Pin 10): Output of the synchronous rectifier and bootstrapped power source for the LTC3422. A ceramic capaci- tor of at least 10 µFisrequiredandshouldbelocatedascloseto VOUT and the power ground plane as possible. Exposed Pad (Pin 11): Signal and Power Ground for the LTC3422. This pin MUST be soldered to the PCB ground plane for electrical contact and rated thermal performance. |
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