Datasheet LT1610 (Analog Devices) - 6

LT1610
U U W U APPLICATIONS INFORMATION OPERATION
If the output load increases sufficiently, A1’s output remains high, resulting in continuous operation. When the LT1610 The LT1610 combines a current mode, fixed frequency is running continuously, peak switch current is controlled PWM architecture with Burst Mode micropower operation by V to maintain high efficiency at light loads. Operation can be C to regulate the output voltage. The switch is turned on at the beginning of each switch cycle. When the sum- best understood by referring to the block diagram in mation of a signal representing switch current and a ramp Figure 2. Q1 and Q2 form a bandgap reference core whose generator (introduced to avoid subharmonic oscillations at loop is closed around the output of the converter. When duty factors greater than 50%) exceeds the V V C signal, IN is 1V, the feedback voltage of 1.23V, along with an comparator A2 changes state, resetting the flip-flop and 70mV drop across R5 and R6, forward biases Q1 and Q2’s turning off the switch. Output voltage increases as switch base collector junctions to 300mV. Because this is not current is increased. The output, attenuated by a resistor enough to saturate either transistor, FB can be at a higher divider, appears at the FB pin, closing the overall loop. voltage than VIN. When there is no load, FB rises slightly Frequency compensation is provided by either an external above 1.23V, causing VC (the error amplifier’s output) to series RC network connected between the V decrease. When V C pin and C reaches the bias voltage on hysteretic ground or the internal RC network on the COMP pin (Pin comparator A1, A1’s output goes low, turning off all 8). The typical values for the internal RC are 50k and 50pF. circuitry except the input stage, error amplifier and low- battery detector. Total current consumption in this state is 30µA. As output loading causes the FB voltage to de-
LAYOUT
crease, A1’s output goes high, enabling the rest of the IC. Although the LT1610 is a relatively low current device, its Switch current is limited to approximately 100mA initially high switching speed mandates careful attention to layout after A1’s output goes high. If the load is light, the output for optimum performance. For boost converters, follow voltage (and FB voltage) will increase until A1’s output the component placement indicated in Figure 3 for the best goes low, turning off the rest of the LT1610. Low fre- results. C2’s negative terminal should be placed close to quency ripple voltage appears at the output. The ripple Pin 4 of the LT1610. Doing this reduces switching currents frequency is dependent on load current and output capaci- in the ground copper which keeps high frequency “spike” tance. This Burst Mode operation keeps the output regu- noise to a minimum. Tie the local ground into the system lated and reduces average current into the IC, resulting in ground plane at one point only, using a few vias, to avoid high efficiency even at load currents of 1mA or less. introducing dI/dt induced noise into the ground plane. GROUND PLANE VIN R1 1 8 C1 + 2 7 R2 LT1610 L1 SHUTDOWN 3 6 4 5 MULTIPLE + VIAs D1 C2 GND VOUT 1610 F03
Figure 3. Recommended Component Placement for Boost Converter. Note Direct High Current Paths Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground Plane. Use Vias at One Location Only to Avoid Introducing Switching Currents into the Ground Plane
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