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LTC3822EDD Datasheet(PDF) 9 Page - Linear Technology |
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LTC3822EDD Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC3822 3822f DUTY CYCLE (%) 10 60 80 110 100 90 3822 F01 40 20 50 70 90 30 10 0 30 50 70 20 0 40 60 80 100 Frequency Selection and Phase-Locked Loop (FREQ Pins) The selection of switching frequency is a tradeoff between efficiency and component size. Low frequency operation increases efficiency by reducing MOSFET switching losses, but requires larger inductance and/or capacitance to main- tain low output ripple voltage. The switching frequency of the LTC3822 is controlled via the FREQ pin. The FREQ pin can be floated, tied to VIN or tied to GND to select 550kHz, 750kHz or 300kHz, respectively. Undervoltage Lockout To prevent operation of the power supply below safe input voltage levels, an undervoltage lockout is incorpo- rated in the LTC3822. When the input supply voltage (VIN) drops below 2.25V, the external MOSFETs and all internal circuits are turned off except for the undervoltage block, which draws only a few microamperes. Peak Current Sense Voltage Selection and Slope Compensation (IPRG Pin) When the LTC3822 controller is operating below 20% duty cycle, the peak current sense voltage (between the VIN and SW pins) allowed across the external top-side MOSFET is determined by: = VA VV SENSE MAX ITH () • –. 07 10 ∆ where A is a constant determined by the state of the IPRG pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN selects A = 5/3; tying IPRG to GND selects A = 2/3. The maximum value of VITH is typically about 1.98V, so the maximum sense voltage allowed across the external P-channel MOSFET is 120mV, 82mV or 200mV for the three respective states of the IPRG pin. However, once the controller’s duty cycle exceeds 20%, slope compensation begins and effectively reduces the peak sense voltage by a scale factor (SF) given by the curve in Figure 1. The peak inductor current is determined by the peak sense voltage and the on-resistance of the external P-channel MOSFET: I V R PK SENSE MAX DS ON = () () ∆ Boost Capacitor Refresh Timeout In order to maintain sufficient charge across CB, the converter will briefly turn off the top MOSFET and turn on the bottom MOSFET if at any time the bottom MOSFET has remained off for 10 switching cycles. This most commonly occurs in a dropout situation. OPERATIO (Refer to Functional Diagram) Figure 1. Maximum Peak Current vs Duty Cycle |
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