LTC3410ESC6-1.875

LTC3410-1.875 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70 U FEATURES DESCRIPTIO ■ The LTC ®3410-1.875 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current during operation is only 26µA, dropping to 1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD). Thus, a 10µF capacitor charging to 3.3V would require a 250µs rise time, limiting the charging current to about 130mA. Therefore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 86.4mW 34101875f 11 LTC3410-1.875 U W U U APPLICATIO S I FOR ATIO PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3410-1.875. These items are also illustrated graphically in Figures 3 and 4. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide. 2. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 3. Keep the (–) plates of CIN and COUT as close as possible. Design Example As a design example, assume the LTC3410-1.875 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.3A but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low 1 and high load currents is important. With this information we can calculate L using Equation (1), L= ⎛ V ⎞ 1 VOUT ⎜ 1− OUT ⎟ ( f)(∆IL ) ⎝ VIN ⎠ Substituting VOUT = 1.875V, VIN = 4.2V, ∆IL = 100mA and f = 2.25MHz in Equation (3) gives: L= 1 . 875V ⎛ 1 . 875V ⎞ 1− = 4 .66µH 2 . 25MHz(100mA) ⎜⎝ 4 . 2V ⎟⎠ A 4.7µH inductor works well for this application. For best efficiency choose a 360mA or greater inductor with less than 0.3Ω series resistance. CIN will require an RMS current rating of at least 0.125A ≅ ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.5Ω. In most cases, a ceramic capacitor will satisfy this requirement. Figure 5 shows the complete circuit along with its efficiency curve. VOUT RUN (3) VIN VIA TO VIN LTC3410-1.875 2 – VOUT GND 6 PIN 1 L1 COUT VOUT + 3 L1 SW VIN 5 LTC34101.875 4 SW CIN VIN COUT CIN 34101875 F03 BOLD LINES INDICATE HIGH CURRENT PATHS Figure 3. LTC3410-1.875 Layout Diagram 34101875 F04 Figure 4. LTC3410-1.875 Suggested Layout 34101875f 12 LTC3410-1.875 U W U U APPLICATIO S I FOR ATIO VIN 2.7V TO 4.2V 4 † CIN 4.7µF CER VIN SW 3 4.7µH* COUT† 4.7µF CER LTC3410-1.875 1 RUN VOUT 6 VOUT 1.875V GND † TAIYO YUDEN JMK212BJ475 *MURATA LQH32CN4R7M23 2, 5 34101875 F05a Figure 5a 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 EFFICIENCY, VIN = 2.7V EFFICIENCY, VIN = 3.6V EFFICIENCY, VIN = 4.2V 1 10 LOAD (mA) 100 1000 34101875 F05b Figure 5b VOUT 100mV/DIV AC COUPLED IL 200mA/DIV ILOAD 200mA/DIV 20µs/DIV VIN = 3.6V ILOAD = 100mA TO 300mA 34101875 F05C Figure 5c 34101875f 13 LTC3410-1.875 U TYPICAL APPLICATIO Using Low Profile Components,