LT3500HDD-PBF

FEATURES n n n n n n n n n n LT3500 Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller DESCRIPTION The LT®3500 is a current mode PWM step-down DC/DC converter with an internal 2.3A switch. The wide input range of 3V to 40V makes the LT3500 suitable for regulating power from a wide variety of sources, including automotive batteries, 24V industrial supplies and unregulated wall adapters. Resistor-programmable 250kHz to 2.2MHz frequency range and synchronization capability enable optimization between efficiency and external component size. Cycleby-cycle current limit, frequency foldback and thermal shutdown provide protection against a shorted output. The soft-start feature controls the ramp rate of the output voltage, eliminating input current surge during start-up, and also provides output tracking. The LT3500 contains an internal NPN transistor with feedback control which can be configured as a linear regulator or as a linear regulator controller. The LT3500’s low current shutdown mode ( 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19 for more information. 3500 F04 Figure 4. Inductor Values for 2A Maximum Load Current (VOUT1 = 3.3V, IRIPPLE = 1A) Input Capacitor Selection Bypass the input of the LT3500 circuit with a 4.7μF or higher ceramic capacitor of X7R or X5R type. A lower value or a less expensive Y5V type can be used if there is additional bypassing provided by bulk electrolytic or tantalum capacitors. The following paragraphs describe the input capacitor considerations in more detail. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3500 and to force this very high frequency switching current into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively, and it must have an adequate ripple current rating. A conservative value is the RMS input current is given by: ICIN(RMS) = IOUT1 ⎡ VOUT1 • ( VIN − VOUT1 ) ⎤ ⎣ ⎦ VIN 0.5 < IOUT1 2 and is largest when VIN = 2VOUT1 (50% duty cycle). 3500fb 14 LT3500 APPLICATIONS INFORMATION The frequency, VIN to VOUT1 ratio, and maximum load current requirement of the LT3500 along with the input supply source impedance, determine the energy storage requirements of the input capacitor. Determine the worstcase condition for input ripple current and then size the input capacitor such that it reduces input voltage ripple to an acceptable level. Typical values for input capacitors run from 10μF at low frequencies to 2.2μF at higher frequencies. The combination of small size and low impedance (low equivalent series resistance or ESR) of ceramic capacitors make them the preferred choice. The low ESR results in very low voltage ripple and the capacitors can handle plenty of ripple current. They are also comparatively robust and can be used in this application at their rated voltage. X5R and X7R types are stable over temperature and applied voltage, and give dependable service. Other types (Y5V and Z5U) have very large temperature and voltage coefficients of capacitance, so they may have only a small fraction of their nominal capacitance in your application. While they will still handle the RMS ripple current, the input voltage ripple may become fairly large, and the ripple current may end up flowing from your input supply or from other bypass capacitors in your system, as opposed to being fully sourced from the local input capacitor. An alternative to a high value ceramic capacitor is a lower value along with a larger electrolytic capacitor, for example a 1μF ceramic capacitor in parallel with a low ESR tantalum capacitor. For the electrolytic capacitor, a value larger than 10μF will be required to meet the ESR and ripple current requirements. Because the input capacitor is likely to see high surge currents when the input source is applied, tantalum capacitors should be surge rated. The manufacturer may also recommend operation below the rated voltage of the capacitor. Be sure to place the 1μF ceramic as close as possible to the VIN and GND pins on the IC for optimal noise immunity. A final caution regarding the use of ceramic capacitors for input bypassing. A ceramic input capacitor can combine with stray inductance to form a resonant tank circuit. If power is applied quickly (for example, by plugging the circuit into a live power source) this tank can ring, doubling the input voltage and damaging the LT3500. The solution is to either clamp the input voltage or dampen the tank circuit by adding a lossy capacitor in parallel with the ceramic capacitor. For details see Application Note 88. Output Capacitor Selection Typically step-down regulators are easily compensated with an output crossover frequency that is 1/10 of the switching frequency. This means that the time that the output capacitor must supply the output load during a transient step is ~2 or 3 switching periods. With an allowable 5% drop in output voltage during the step, a good starting value for the output capacitor can be expressed by: C VOUT1 = Example: VOUT1 = 3.3V, Frequency = 1MHz, Max Load Step = 2A C VOUT1 = 2 = 12μF 1MHz • 0.05 • 3.3 Max Load Step Frequency • 0.05 • VOUT1 The calculated value is only a suggested starting value. Increase the value if transient response needs improvement or reduce the capacitance if size is a priority. The output capacitor filters the inductor current to generate an output with low voltage ripple. It also stores energy in order to satisfy transient loads and to stabilize the LT3500’s control loop. The switching frequency of the LT3500 determines 3500fb 15 LT3500 APPLICATIONS INFORMATION the value of output capacitance required. Also, the current mode control loop doesn’t require the presence of output capacitor series resistance (ESR). For these reasons, you are free to use ceramic capacitors to achieve very low output ripple and small circuit size. Estimate output ripple with the following equations: VRIPPLE = ΔIL 8 • Frequency • COUT1 The high performance (low ESR), small size and robustness of ceramic capacitors make them the preferred type for LT3500 applications. However, all ceramic capacitors are not the same. As mentioned above, many of the high value capacitors use poor dielectrics with high temperature and voltage coefficients. In particular, Y5V and Z5U types lose a large fraction of their capacitance with applied voltage and temperature extremes. Because the loop stability and transient response depend on the value of COUT1, you may not be able to tolerate this loss. Use X7R and X5R types. You can also use electrolytic capacitors. The ESRs of most aluminum electrolytics are too large to deliver low output ripple. Tantalum and newer, lower ESR organic electrolytic capacitors intended for power supply use, are suitable and the manufacturers will specify the ESR. The choice of capacitor value will be based on the ESR required for low ripple. Because the volume of the capacitor determines its ESR, both the size and the value will be larger than a ceramic capacitor that would give you similar ripple performance. One benefit is that the larger capacitance may give better transient response for large changes in load current. Catch Diode The diode D1 conducts current only during switch off time. Use a Schottky diode to limit forward voltage drop to increase efficiency. The Schottky diode must have a peak reverse voltage that is equal to regulator input voltage and sized for average forward current in normal operation. Average forward current can be calculated from: ID(AVG) = IOUT1 • ( VIN − VOUT1) VIN For ceramic capacitors and, VRIPPLE = ΔIL • ESR For electrolytic (tantalum and aluminum) where ΔIL is the peak-to-peak ripple current in the inductor. The RMS content of this ripple is very low, and the RMS current rating of the output capacitor is usually not of concern. Another constraint on the output capacitor is that it must have greater energy storage than the inductor; if the stored energy in the inductor is transferred to the output, you would like the resulting voltage step to be small compared to the regulation voltage. For a 5% overshoot, this requirement becomes: I COUT1 > 10 • L LIM VOUT1 2 Finally, there must be enough capacitance for good transient performance. The last equation gives a good starting point. Alternatively, you can start with one of the designs in this data sheet and experiment to get the desired performance. This topic is covered more thoroughly in the section on loop compensation. 3500fb 16 LT3500 APPLICATIONS INFORMATION The only reason to consider a larger diode is the worstcase condition of a high input voltage and shorted output. With a shorted condition, diode current will increase to a typical value of 3A, determined by the peak switch current limit of the LT3500. This is safe for short periods of time, but it would be prudent to check with the diode manufacturer if continuous operation under these conditions can be tolerated. BST Pin Considerations The capacitor and diode tied to the BST pin generate a voltage that is higher than the input voltage. In most cases a 0.47μF capacitor and fast switching diode (such as the CMDSH-3 or FMMD914) will work well. Almost any type of film or ceramic capacitor is suitable, but the ESR should be VIN + 3V VOUT1 VOUT1 VBST – VSW = VX VBST(MAX) = VX VBST – VSW = VOUT2 VBST(MAX) = VIN + VOUT2 VOUT2 ≥ 2.5V 3500 F05 (5c) (5d) Figure 5. BST Pin Considerations 3500fb 17 LT3500 APPLICATIONS INFORMATION The boost circuit can also run directly from a DC voltage that is higher than the input voltage by more than 2.5V, as in Figure 5d. The diode is used to prevent damage to the LT3500 in case VX is held low while VIN is present. The circuit eliminates a capacitor, but efficiency may be lower and dissipation in the LT3500 may be higher. Also, if VX is absent, the LT3500 will still attempt to regulate the output, but will do so with very low efficiency and high dissipation because the switch will not be able to saturate, dropping 1.5V to 2V in conduction. The minimum input voltage of an LT3500 application is limited by the minimum operating voltage (