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LTM4653 Datasheet(PDF) 15 Page - Linear Technology |
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LTM4653 Datasheet(HTML) 15 Page - Linear Technology |
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15 / 32 page  LTM4653 15 Rev 0 For more information www.analog.com OPERATION approximately –1A, allowing for significant negative aver- age current. Clocking the CLKIN pin at a frequency within ±40% of the target switching frequency commanded by the fSET pin synchronizes MT’s turn-on to the rising edge of the CLKIN pin. Output Voltage Programming, Tracking and Soft-Start The LTM4653 regulates its output voltage, VOUT, accord- ing to the differential voltage present across ISETa and SGND. In most applications, the output voltage is set by simply connecting a resistor, RISET, from ISETa to SGND, according to: RISET = VOUT 50µA Since the LTM4653 control loop servos its output volt- age according to the voltage between ISETa and SGND: placing a capacitor, CSS, parallel to RSET configures the ramp-up rate of ISETa and thus VOUT. In the time domain, the output voltage ramp-up after the RUN pin is toggled from low to high (t = 0s) is given by: VOUT(t)=IISETa •RISET • 1– e – t RISET • CSS ⎛ ⎝ ⎜ ⎜⎜ ⎞ ⎠ ⎟ ⎟⎟ The soft-start time, tSS, is defined as the time it takes for VOUT to ramp from 0V to 90% of its final value: tSS = –RISET •CSS •In (1– 0.9) or tSS = 2.3 • RISET •CSS A default value of CSS = 1.5nF can be implemented by connecting ISETa to ISETb. For other ramp-up rates, connect an external CSS capacitor parallel to RISET. When starting up into a pre-biased VOUT, the LTM4653 stays in a sleep mode, keeping MT and MB off until VISETa equals VOSNS—after which, the DC/DC converter commences switching action and VOUT is ramped according to the voltage commanded by ISETa. Since the LTM4653 control loop servos its VOSNS voltage to match that of ISETa’s, the LTM4653’s output can be configured to track any voltage applied to ISETa, refer- enced to SGND. Frequency Adjustment The default switching frequency (fSW) of the LTM4653 is 400kHz. This is suitable for low-VIN (VIN ≤ 5V) ap- plications and low-VOUT (VOUT≤3.3V) applications. For a practical design, the LTM4653’s inductor ripple current (∆IPK-PK) is suggested to be less than ~2APK-PK. Choose fSW according to: fSW = VOUT • 1− D ( ) L • ∆IPK-PK where the value of LTM4653’s power inductor, L, is 4μH. To avoid cycle-skipping, impose restrictions on fSW, to ensure minimum on time criteria is met: fSW < D TON(MIN) The LTM4653’s minimum on-time, tON(MIN), is specified as 60ns. For a practical design, it is recommended to guardband to 90ns. ToconfiguretheLTM4653forahigherswitchingfrequency than its default of 400kHz, apply a resistor, RfSET, between the fSET pin and SGND. RfSET is given (in MΩ) by: RfSET(MΩ)= 1 10pF •[fSW(MHz)– 0.4(MHz)] The relationship of RfSET to programmed fSW is shown in Figure 1. See Table 7 for recommended fSW and cor- responding RfSET values for various combinations of VIN and VOUT. RfSET NOT USED RfSET (kΩ) 10 100 1k 10k 0.1 1 10 4653 F01 Figure 1. Relationship Between RfSET and Target fSW |
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