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LTC1143 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1143 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1143/LTC1143L LTC1143L-ADJ OPERATION Refer to Functional Diagram and Figure 1 The circuit now enters sleep mode with the power MOSFET turned off. In sleep mode a majority of the circuitry is turned off, dropping the quiescent current from 1.6mA to 160 µA (for one regulator block). The load current is now being supplied from the output capacitor. When the output voltage has dropped by the amount of hysteresis in comparator V, the P-channel MOSFET is again turned on and the process repeats. To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset (VOS) is incorpo- rated in the gain stage. This prevents the current compara- tor threshold from increasing until the output voltage has dropped below a minimum threshold. Using constant off-time architecture the operating fre- quency is a function of the input voltage. To minimize the frequency variation as dropout is approached, the off-time controller increases the CT discharge current as VIN drops below VOUT + 1.5V. In dropout the P-channel MOSFET is turned on continuously (100% duty cycle), providing extremely low dropout operation. For Figure 1 applications with VOUT below 2V, or when RSENSE is moved to ground, the current sense comparator inputs operate near ground. When the current comparator is operated at less than 2V common mode, the off-time increases approximately 40%, requiring the use of a smaller timing capacitor CT. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1143 series current comparators have a threshold range that extends from a minimum of 25mV/RSENSE to a maximum of 150mV/RSENSE. The current comparator threshold sets the peak of the inductor ripple current, yielding a maximum output current IMAX equal to the peak value less half the peak-to-peak ripple current. For proper Burst Mode operation, IRIPPLE(P-P) must be less than or equal to the minimum current comparator threshold. Since efficiency generally increases with ripple current, the maximum allowable ripple current is assumed, i.e., IRIPPLE(P-P) = 25mV/RSENSE. (See CT and L Selection for Operating Frequency). Solving for RSENSE and allowing a margin for variations in the LTC1143 series and external component values yields: RSENSE = 100mV IMAX A graph for selecting RSENSE versus maximum output current is given in Figure 2. The basic LTC1143L-ADJ application circuit is shown in Figure 1. The LTC1143 and LTC1143L are similar but omit the external resistive VOUT dividers (see Figures 10 and 13). External component selection is driven by the load requirement and begins with VOUT and the selection of RSENSE. Once RSENSE is known, CT and L can be chosen. Next, the power MOSFET and D1 are selected. Finally, CIN and COUT are selected and the loop is compensated. Since the two regulator sections are identical, the process of component selection is the same for both sections. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 16V. Output Voltage Selection The LTC1143/LTC1143L output voltages are internally set to 3.3V and 5V. The LTC1143L-ADJ requires an external resistive divider from VOUT to VFB on each section as shown in Figure 1. The regulated LTC1143L-ADJ output voltages are given by: V R R V R R OUT OUT 1 2 125 1 2 1 125 1 4 3 =+ =+ . . To prevent stray pickup, a 100pF capacitor is suggested across R1 and R3 located close to the LTC1143L-ADJ. APPLICATIONS INFORMATION |
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Similar Description - LTC1143 |
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