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Both parts include a 1A integrated switch allowing high current outputs to be generated in a small footprint. The LT switches at 1. These high speeds enable the use of tiny, low cost capacitors and inductors 2mm or less in height.
Both parts are easy pin-for-pin upgrades for higher power LT applications. Very low output voltage ripple approaching 1mVP-P can be achieved when ceramic output capacitors are used. Fixed frequency switching ensures a clean output free from low frequency noise typically present with charge pump solutions.
F L1B 10? H 95 D1 SW R1 Minimize trace area at this pin to keep EMI down. GND Pin 2 : Ground. Tie directly to local ground plane. Reference voltage is —1. Connect resistive divider tap here. Minimize trace area. The NFB bias current flows out of the pin. Tie to 2. Ground to shut down. Must be locally bypassed. Operation can be best understood by referring to the Block Diagram in Figure 2. At the start of each oscillator cycle, the SR latch is set, turning on the power switch Q3.
A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset, turning off the power switch.
The level at the negative input of A2 is set by the error amplifier gm and is simply an amplified version of the difference between the feedback voltage and the reference voltage of —1. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation.
One function not shown in Figure 2 is the current limit. The switch current is constantly monitored and not allowed to exceed the nominal value of 1. If the switch current reaches 1. This current limit protects the power switch as well as various external components connected to the LT Care must be taken in deciding which part to use. The high switching frequency of the LTA allows smaller cheaper inductors and capacitors to be used in a given application, but with a slight decrease in efficiency and maximum output current when compared to the LT Generally, if efficiency and maximum output current are critical, the LT should be used.
If application size and cost are more important, the LTA will be the better choice. In many applications, tiny inexpensive chip inductors can be used with the LTA, reducing solution cost. A 5V to —16V application has a DC of However, the part must be operated in the discontinuous conduction mode so that the actual duty cycle is reduced. Besides these, there are many other inductors that can be used. Consult each manufacturer for detailed information and for their entire selection of related parts.
Ferrite core inductors should be used to obtain the best efficiency, as U core losses at frequencies above 1MHz are much lower for ferrite cores than for powdered-iron units. When using coupled inductors, choose one that can handle at least 1A of current without saturating, and ensure that the inductor has a low DCR copper-wire resistance to minimize I2R power losses. If using uncoupled inductors, each inductor need only handle one-half of the total switch current so that 0.
H to 15? H coupled inductor or a 15? H to 22? H uncoupled inductor will usually be the best choice for most LT designs. For the LTA, a 2. H coupled inductor or a 3. H to 10? H uncoupled inductor will usually suffice. In this case, the inductor must carry the entire 1A switch current. Table 1. For better efficiency, use similar valued inductors with a larger volume.
For instance, the Sumida CR43 series, in values ranging from 3. H, will give a LTA application a few percentage points increase in efficiency. Multilayer ceramic capacitors are an excellent choice, as they have an extremely low ESR and are available in very small packages. X5R dielectrics are preferred, followed by X7R, as these materials retain their capacitance over wide voltage and temperature ranges.
F to 22? F output capacitor is sufficient for most LT applications while a 4. F to 10? F capacitor will suffice for the LTA. Always use a capacitor with a sufficient voltage rating. F input capacitor is sufficient for most applications. Table 3 shows a list of several ceramic capacitor manufacturers.
Consult the manufacturers for detailed information on their entire selection of ceramic parts. Table 3. The ESR of any capacitor, along with the capacitance itself, contributes a 6 U zero to the system. For the tantalum and OS-CON capacitors, this zero is located at a lower frequency due to the higher value of the ESR, while the zero of a ceramic capacitor is at a much higher frequency and can generally be ignored.
The frequency of the zero is determined by the following equation. C4 By choosing the appropriate values for the resistor and capacitor, the zero frequency can be designed to improve the phase margin of the overall converter.
The typical target value for the zero frequency is between 20kHz to 60kHz. Figure 3 shows the transient response of the inverting converter from Figure 1 without the phase lead capacitor C4. The phase margin is reduced as evidenced by more ringing in both the output voltage and inductor current. A pF capacitor for C4 results in better phase margin, which is revealed in Figure 4 as a more damped response and less overshoot. Figure 5 shows the transient response when a 22? F tantalum capacitor with no phase lead capacitor is used on the output.
The higher output voltage ripple is revealed in the upper waveform as a thicker line. The transient response is adequate which implies that the ESR zero is improving the phase margin. Transient Response of Inverting Converter with 22? This is an inherent feature of switching regulators in general since the feedback loop is saturated due to VOUT being far from its final value. No Soft-Start Circuit. Figure 6 shows a typical oscillograph of the start-up waveform for the application of Figure 1 starting into a load of 33?.
The middle waveform shows the input current, which reaches as high as 0. The total time required for the output to reach its final value is approximately ? For some applications, this initial inrush current may not be acceptable.
If a longer start-up time is acceptable, a soft-start circuit consisting of RSS and CSS, as shown in Figure 7, can be used to limit inrush current to a lower value. Input current, measured at VIN, is limited to a peak value of 0. Suggested Component Placement. The Motorola MBR is a very good choice. These diodes are rated to handle an average forward current of 0. In applications where the average forward current of the diode exceeds 0.
You will not get advertised performance with careless layout. Figure 10 shows the recommended component placement. The ground cut at the cathode of D1 is essential for low noise operation. F L2 10? F 16V R1 1? 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.
F 25V C4 1? F L1B 4.
Sumida Corporation CR43NP-100MC
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