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LT1612ES8 Datasheet(PDF) 6 Page - Linear Technology |
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LT1612ES8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LT1612 sn1612 1612fs Figure 2. Recommended Component Placement. Traces Carrying High Current are Direct. Trace Area at FB Pin and VC Pin Is Kept Low. Lead Length to Battery Should Be Kept Short 1 2 8 7 3 4 6 5 L1 C2 R1 C3 LT1612 VOUT VIN GND SHDN 1612 F02 MODE MULTIPLE VIAs C1 R2 RC CC OPERATIO The LT1612 employs fixed frequency, current mode con- trol. This type of control uses two feedback loops. The main control loop sets output voltage and operates as follows: A load step causes VOUT and the FB voltage to be perturbed slightly. The error amplifier responds to this change in FB by driving the VC pin either higher or lower. Because switch current is proportional to the VC pin voltage, this change causes the switch current to be adjusted until VOUT is once again satisfied. Loop compen- sation is taken care of by an RC network from the VC pin to ground. Inside this main loop is another that sets current limit on a cycle-by-cycle basis. This loop utilizes current compara- tor A2 to control peak current. The oscillator runs at 800kHz and issues a set pulse to the flip-flop at the beginning of each cycle, turning the switch on. With the switch now in the ON state the SW pin is effectively connected to VIN. Current ramps up in the inductor linearly at a rate of (VIN – VOUT)/L. Switch current is set by the VC pin voltage and when the voltage across RSENSE trips the current comparator, a reset pulse will be generated and the switch will be turned off. Since the inductor is now loaded up with current, the SW pin will fly low and trigger the rectifier to turn on. Current will flow through the rectifier decreasing at a rate of VOUT/L until the oscillator issues a new set pulse, causing the cycle to repeat. If the load is light and VC decreases below A3’s trip point, the device will enter the Burst Mode operation region (the MODE pin must be at ground or floating). In this state the oscillator and all other circuitry except the reference and comparator A3 are switched on and off at low frequency. This mode of operation increases efficiency at light loads but introduces low frequency voltage ripple at the output. For continuous switching and no low frequency output voltage ripple, pull the MODE pin high. This will disable comparator A3 which forces the oscillator to run continuously. Layout Hints The LT1612 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layout. Figure 2 shows recommended component placement for a buck (step-down) converter. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor C1 must be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation. The ground terminal of input capacitor C1 should tie close to Pin 4 of the LT1612. Doing this reduces dI/dt in the ground copper which keeps high frequency spikes to a minimum. The DC/DC converter ground should tie to the PC board ground plane at one place only, to avoid intro- ducing dI/dt in the ground plane. |
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