LTC3409AEDD

LTC3409A 600mA Low VIN Buck Regulator in 3mm × 3mm DFN FEATURES n n n n n n n n n n n n n n n n n DESCRIPTION The LTC®3409A is a high efficiency, monolithic synchronous buck regulator using a constant frequency, current mode architecture. The LTC3409A improves upon the LTC3409’s light load regulation in Burst Mode operation. The output voltage is adjusted via an external resistor divider. Fixed switching frequencies of 1.7MHz and 2.6MHz are supported. Alternatively, an internal PLL will synchronize to an external clock in the frequency range of 1MHz to 3MHz. This range of switching frequencies allows the use of small surface mount inductors and capacitors, including ceramics. Supply current during Burst Mode operation is only 65μA 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. Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3409A. These items are also illustrated graphically in the layout diagram of Figure 3. Check the following in your layout. 1. Does the capacitor CIN connect to the power VIN (Pins 3, 4) and GND (Exposed Pad) as close as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers. 2. Are the COUT and L1 closely connected? The (–) plate of COUT returns current to GND and the (–) plate of CIN. 3. The resistor divider, R1 and R2, must be connected between the (+) plate of COUT and a ground sense line terminated near GND (Exposed Pad). The feedback signals VFB should be routed away from noisy components and traces, such as the SW line (Pins 6), and its trace should be minimized. 4. The SW trace should be kept as small as possible. Keep sensitive components away from the SW pins. The input capacitor CIN and the resistors R1 and R2 should be routed away from the SW traces and the inductors. 5. A ground plane is preferred, but if not available, keep the signal and power grounds segregated with small signal components returning to the GND pin at one point. They should not share the high current path of CIN or COUT. 6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. These copper areas should be connected to VIN or GND (preferably). VIN CIN VIN RUN VFB VIN SYNC MODE SW SGND GND C1 L1 VOUT LTC3409A COUT R2 R1 3409A F03 Figure 3 3409af 13 LTC3409A APPLICATIONS INFORMATION Design Example As a design example, assume the LTC3409A is used in a 2-alkaline cell battery-powered application. The VIN will be operating from a maximum of 3.2V down to about 1.8V. The load current requirement is a maximum of 600mA but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important, so the minimum frequency setting of 1.7MHz is chosen. Output voltage is 1.5V. With this information we can calculate L using Equation 3: ⎛V⎞ 1 L= VOUT ⎜ 1– OUT ⎟ f • ΔIL VIN ⎠ ⎝ (3) For best efficiency choose a 750mA or greater inductor with less than 0.3Ω series resistance. CIN will require an RMS current rating of at least 0.3A ≅ ILOAD(MAX) /2 at temperature. For the feedback resistors, choose R2 = 137k. R1 is then calculated to be 200k from Equation 2. Figure 4 shows the complete circuit along with its efficiency curve. Table 2 below gives 1% resistor values for selected output voltages. VOUT 0.85V 1.2V 1.5V 1.8V R1 53.6k 137k 200k 267k R2 137k 143k 137k 137k Substituting VOUT = 1.5V, VIN = 3.2V, ΔIL = 240mA and f = 1.7MHz in Equation 2 gives: L= 1 ⎛ 1.5V ⎞ 1.5V ⎜ 1– ≅ 2.2µH ⎝ 3.2V ⎟ ⎠ 1.7MHz • 240mA Burst Mode Efficiency, 1.5VOUT VIN 1.8V TO 3.2V CIN 4.7μF 100 R2 137k 90 1.8VIN 3.2VIN LTC3409A VFB GND VIN VIN R1 200k SYNC RUN SW MODE 3409A F04 80 EFFICIENCY (%) L1 2.2μH COUT 22μF 2 70 60 50 40 30 20 2.5VIN VOUT 1.5V 0.6A L1: SUMIDA CDRH2D18/LD C1 10pF 10 0 0.1 1 10 100 LOAD CURRENT (mA) 1000 3409A F04b Figure 4 3409af 14 LTC3409A PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) 0.675 ± 0.05 3.5 ± 0.05 1.65 ± 0.05 2.15 ± 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ± 0.05 (2 SIDES) R = 0.115 TYP 5 0.38 ± 0.10 8 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ± 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 1.65 ± 0.10 (2 SIDES) (DD) DFN 1203 0.200 REF 0.75 ± 0.05 4 0.25 ± 0.05 2.38 ± 0.10 (2 SIDES) 1 0.50 BSC 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON TOP AND BOTTOM OF PACKAGE 3409af Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC3409A RELATED PARTS PART NUMBER LTC3409 LTC3549 LTC3417A-2 LTC3410 LTC1878 LTC1879 LT3020 LTC3025 LTC3404 LTC3405A LTC3406A/ LTC3406AB LTC3407A/ LTC3407A-2 LTC3411A DESCRIPTION 600mA (IOUT) Low VIN Step-Down DC/DC Converter 250mA Low VIN Step-Down DC/DC Converter Dual 1.5A/1A 4MHz Step-Down DC/DC Regulator 300mA, 2.25MHz, Synchronous Step-Down Regulator in SC-70 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.20A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 100mA, Low Voltage VLDO™ 100mA, Low Voltage VLDO 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter Dual, 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter COMMENTS 95% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.61V, IQ = 65μA, ISD < 1μA, 8-Lead DFN Package 93% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.61V, IQ = 50μA, ISD < 1μA, 6-Lead DFN Package 95% Efficiency, Low Ripple, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, DFN and TSSOP Packages 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26μA 96% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 10μA, ISD