Datasheet LTC3405A-1.375 (Analog Devices) - 10

LTC3405A-1.375
U U W U APPLICATIO S I FOR ATIO
where PD is the power dissipated by the regulator and θJA
PC Board Layout Checklist
is the thermal resistance from the junction of the die to the When laying out the printed circuit board, the following ambient temperature. checklist should be used to ensure proper operation of the The junction temperature, TJ, is given by: LTC3405A-1.375. These items are also illustrated graphi- T cally in Figures 3 and 4. Check the following in your layout: J = TA + TR where T 1. The power traces, consisting of the GND trace, the SW A is the ambient temperature. trace and the VIN trace should be kept short, direct and As an example, consider the LTC3405A-1.375 with an wide. input voltage of 2.7V, a load current of 300mA and an ambient temperature of 70°C. From the typical perfor- 2. Does the (+) plate of CIN connect to VIN as closely as mance graph of switch resistance, the R possible? This capacitor provides the AC current to the DS(ON) of the P- channel switch at 70°C is approximately 0.94Ω and the internal power MOSFETs. RDS(ON) of the N-channel synchronous switch is approxi- 3. Keep the (–) plates of CIN and COUT as close as possible. mately 0.75Ω. The series resistance looking into the SW pin is: RSW = 0.95Ω (0.51) + 0.75Ω (0.49) = 0.85Ω 1 6 RUN MODE Therefore, power dissipated by the part is: LTC3405A-1.375 2 5 P 2 GND – VOUT D = ILOAD • RSW = 76.5mW VOUT COUT For the SOT-23 package, the θ 3 4 JA is 250°C/ W. Thus, the + SW VIN L1 junction temperature of the regulator is: CIN V T IN J = 70°C + (0.0765)(250) = 89.1°C 3405A1375 F03 which is well below the maximum junction temperature of BOLD LINES INDICATE HIGH CURRENT PATHS 125°C.
Figure 3. LTC3405A-1.375 Layout Diagram
Note that at higher supply voltages, the junction tempera- ture is lower due to reduced switch resistance (R
Design Example
DS(ON)). As a design example, assume the LTC3405A-1.375 is
Checking Transient Response
used in a single lithium-ion battery-powered cellular phone The regulator loop response can be checked by looking at application. The VIN will be operating from a maximum of the load transient response. Switching regulators take 4.2V down to about 2.7V. The load current requirement several cycles to respond to a step in load current. When is a maximum of 0.15A but most of the time it will be in a load step occurs, VOUT immediately shifts by an amount standby mode, requiring only 2mA. Efficiency at both low equal to (∆ILOAD • ESR), where ESR is the effective series and high load currents is important. Output voltage is resistance of COUT. ∆ILOAD also begins to charge or 1.375V. With this information we can calculate L using discharge COUT, which generates a feedback error signal. equation (1), The regulator loop then acts to return VOUT to its steady- 1 ⎛ ⎞ state value. During this recovery time V V OUT can be moni- L = V OUT OUT ⎜1 ⎟ tored for overshoot or ringing that would indicate a stability f I ⎝ V ( ) ∆ ( (3) L IN ) − ⎠ problem. For a detailed explanation of switching control loop theory, see Application Note 76. 3405a1375f 10