SNVS075E May 2001 – May 2016 LM2592HV
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Description (continued)
- 6 Pin Configuration and Functions
-
7 Specifications
- 7.1 Absolute Maximum Ratings
- 7.2 ESD Ratings
- 7.3 Recommended Operating Conditions
- 7.4 Thermal Information
- 7.5 Electrical Characteristics LM2592HV-3.3
- 7.6 Electrical Characteristics LM2592HV-5.0
- 7.7 Electrical Characteristics LM2592HV-ADJ
- 7.8 Electrical Characteristics All Output Voltage Versions
- 7.9 Typical Characteristics
- 8 Parameter Measurement Information
- 9 Detailed Description
- 10Application and Implementation
- 11Power Supply Recommendations
- 12Layout
- 13Device and Documentation Support
- 14Mechanical, Packaging, and Orderable Information
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10 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
10.1 Application Information
10.1.1 Feedforward Capacitor
(Adjustable Output Voltage Version)
CFF – A feedforward Capacitor CFF, shown across R2 in Test Circuits, is used when the output voltage is greater than 10 V or when COUT has a very low ESR. This capacitor adds lead compensation to the feedback loop and increases the phase margin for better loop stability.
If the output voltage ripple is large (>5% of the nominal output voltage), this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation, adding a resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at the feedback pin.
10.1.2 Input Capacitor
CIN – A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It must be located near the regulator using short leads. This capacitor prevents large voltage transients from appearing at the input, and provides the instantaneous current needed each time the switch turns on.
The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of the relatively high RMS currents flowing in a buck regulator's input capacitor, this capacitor must be chosen for its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage rating are directly related to the RMS current rating. The voltage rating of the capacitor and its RMS ripple current capability must never be exceeded.
10.1.3 Output Capacitor
COUT – An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters are the 100-kHz Equivalent Series Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter. The ESR must generally not be less than 100 mΩ or there will be loop instability. If the ESR is too large, efficiency and output voltage ripple are effected, so ESR must be chosen carefully.
10.1.4 Catch Diode
Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This must be a fast diode and must be located close to the LM2592HV using short leads and short printed-circuit traces.
Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best performance, especially in low output voltage applications (5 V and lower). Ultra-fast recovery, or high-efficiency rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. The diode must be chosen for its average/RMS current rating and maximum voltage rating. The voltage rating of the diode must be greater than the DC input voltage (not the output voltage).
10.1.5 Inverting Regulator
The circuit in Figure 24 converts a positive input voltage to a negative output voltage with a common ground. The circuit operates by bootstrapping the regulator's ground pin to the negative output voltage. Then, after grounding the feedback pin, the regulator senses the inverted output voltage and regulates it.
This example uses the LM2592HV-5.0 to generate a −5-V output, but other output voltages are possible by selecting other output voltage versions, including the adjustable version. Because this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maximum output current greatly depends on both the input and output voltage.
To determine how much load current is possible before the internal device current limit is reached (and power limiting occurs), the system must be evaluated as a buck-boost configuration rather than as a buck. The peak switch current in amperes, for such a configuration is given as Equation 1:
where
- L is in μH
- and f is in Hz
- The maximum possible load current ILOAD is limited by the requirement that IPEAK ≤ ICLIM
While checking for this, take ICLIM to be the lowest possible current limit value (minimum across tolerance and temperature is 2.3 A for the LM2592HV). Also to account for inductor tolerances, take the minimum value of Inductance for L in Equation 1 (typically 20% less than the nominal value). Further, Equation 1 disregards the drop across the switch and the diode. This is equivalent to assuming 100% efficiency, which is never so. Therefore expect IPEAK to be an additional 10 to 20% higher than calculated from the above equation.
See Application Note AN-1157 for examples based on positive to negative configuration.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 60 V. For example, when converting +20 V to −12 V, the regulator would see 32 V between the input pin and ground pin. The LM2592HV has a maximum input voltage spec of 60 V.
Additional diodes are required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode isolation changes the topology to closely resemble a buck configuration thus providing good closed loop stability. A Schottky diode is recommended for low input voltages, (because of its lower voltage drop) but for higher input voltages, a fast recovery diode could be used.
Without diode D3, when the input voltage is first applied, the charging current of CIN can pull the output positive by several volts for a short period of time. Adding D3 prevents the output from going positive by more than a diode voltage.
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to select the inductor value. In the majority of designs, a 33-μH, 4-A inductor is the best choice. Capacitor selection can also be narrowed down to just a few values.
This type of inverting regulator can require relatively large amounts of input current when starting up, even with light loads. Input currents as high as the LM2592HV current limit (approximately 4 A) are needed for at least 2 ms or more, until the output reaches its nominal output voltage. The actual time depends on the output voltage and the size of the output capacitor. Input power sources that are current-limited or sources that can not deliver these currents without getting loaded down, may not work correctly. Because of the relatively high start-up currents required by the inverting topology, the delayed start-up feature (C1, R1, and R2) shown in Figure 24 is recommended. By delaying the regulator start-up, the input capacitor is allowed to charge up to a higher voltage before the switcher begins operating. A portion of the high input current needed for start-up is now supplied by the input capacitor CIN). For severe start-up conditions, the input capacitor can be made much larger than normal.
10.1.6 Inverting Regulator Shutdown Methods
Using the ON/OFF pin in a standard buck configuration is simple. To turn the regulator ON, pull the ON/OFF pin below 1.3 V (at 25°C referenced to ground). To shut the regulator OFF, pull the ON/OFF pin above 1.3 V. With the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now setting at the negative output voltage level. Two different shutdown methods for inverting regulators are shown in Figure 25 and Figure 26.
Figure 25. Inverting Regulator Ground Referenced Shutdown
10.2 Typical Application
Figure 27. Typical Application
10.2.1 Design Requirements
Table 1 lists the parameters for this design example.
Table 1. Example Parameters
| PARAMETER | EXAMPLE VALUE |
|---|---|
| Regulated output voltage, VOUT | 20 V |
| Maximum input voltage, VIN(max) | 24 V |
| Maximum load current, ILOAD(max) | 1 A |
| Switching frequency, F | Fixed at a nominal 150 kHz |
10.2.2 Detailed Design Procedure
10.2.2.1 Inductor Selection Procedure
See application note AN-1197 (SNVA038) for detailed information on selecting inductors for buck converters. For a quick-start, the designer may refer to the nomographs provided in Figure 28 to Figure 30. To give designers more options of available inductors, the nomographs provide the required inductance and also the energy in the core expressed in microjoules (µJ), as an alternative to just prescribing custom parts. The following points must be highlighted:
- The Energy values shown on the nomographs apply to steady operation at the corresponding x-coordinate (rated maximum load current). However, under start-up, without soft start, or a short circuit on the output, the current in the inductor will momentarily/repetitively hit the current limit ICLIM of the device, and this current could be much higher than the rated load, ILOAD. This represents an overload situation, and can cause the inductor to saturate (if it has been designed only to handle the energy of steady operation). However, most types of core structures used for such applications have a large inherent air gap (for example, powdered iron types or ferrite rod inductors), so the inductance does not fall off too sharply under an overload. The device is usually able to protect itself by preventing the current from exceeding ICLIM. However, if the DC input voltage of the regulator is over 40 V, the current can slew up so fast under core saturation that the device may not be able to act fast enough to restrict the current. The current can then rise without limit until the device destructs. Therefore to ensure reliability, it is recommended, that if the DC Input Voltage exceeds 40 V, the inductor must ALWAYS be sized to handle an instantaneous current equal to ICLIM without saturating, irrespective of the type of core structure/material.
- The energy under steady operation is calculated in Equation 2:
- L is in µH
- and IPEAK is the peak of the inductor current waveform with the regulator delivering ILOAD
- The energy under overload is calculated in Equation 3:
- The nomographs were generated by allowing a greater amount of percentage current ripple in the inductor as the maximum rated load decreases (see Figure 31). This was done to allow smaller inductors to be used at light loads. Figure 31, however, shows only the median value of the current ripple. In reality there may be a great spread around this because the nomographs approximate the exact calculated inductance to standard available values. It is a good idea to refer to AN-1197 for detailed calculations if a certain maximum inductor current ripple is required for various possible reasons. Also consider the rather wide tolerance on the nominal inductance of commercial inductors.
- Figure 30 shows the inductor selection curves for the Adjustable version. The y-axis is 'Et', in Vμs. It is the applied volts across the inductor during the ON time of the switch (VIN – VSAT – VOUT) multiplied by the time for which the switch is on in μs. See Example 3.
where
These are the energy values shown in the nomographs. See Example 1.
If VIN > 40 V, the inductor must be sized to handle eCLIM instead of the steady energy values. The worst case ICLIM for the LM2592HV is 4 A. The energy rating depends on the Inductance. See Figure 28 through Figure 30.
Example 1: (VIN ≤ 40 V) LM2592HV-5.0, VIN = 24 V, Output 5 V at 1 A
- A first pass inductor selection is based upon Inductance and rated max load current. Choose an inductor with the inductance value indicated by the nomograph (see Figure 29) and a current rating equal to the maximum load current. We therefore quick-select a 68-μH, 1-A inductor (designed for 150-kHz operation) for this application.
- Confirm that it is rated to handle 50 μJ (see Figure 29) by either estimating the peak current or by a detailed calculation as shown in AN-1197 (SNVS038). Also, confirm that the losses are acceptable.
Example 2: (VIN > 40 V) LM2592HV-5.0, VIN = 48 V, Output 5 V at 1.5 A
- A first pass inductor selection is based upon Inductance and the switch currrent limit. We choose an inductor with the Inductance value indicated by the nomograph (see Figure 29) and a current rating equal to ICLIM. We therefore quick-select a 68-μH, 4-A inductor (designed for 150-kHz operation) for this application.
- Confirm that it is rated to handle eCLIM by the procedure shown in AN-1197 (SNVA038) and that the losses are acceptable. Here eCLIM is calculated in Equation 4:
Example 3: (VIN ≤ 40 V) LM2592HV-ADJ, VIN = 20 V, Output 10 V at 2 A
- Because input voltage is less than 40 V, a first pass inductor selection is based upon inductance and rated maximum load current. Choose an inductor with the inductance value indicated by the nomograph Figure 30 and a current rating equal to the maximum load. But first calculate Et for the given application. The duty cycle is calculated in Equation 5:
- VD is the drop across the catch diode (≊ 0.5 V for a Schottky)
- and VSAT the drop across the switch (≊ 1.5 V)
- f is the switching frequency in Hz
- Confirm that the inductor is rated to handle 200 μJ (see Figure 30) by the procedure shown in AN-1197 (SNVA038) and that the losses are acceptable. (If the DC input voltage is greater than 40 V, consider eCLIM as shown in Example 2.)
where
Which turns into Equation 6
And the switch ON time is calculated by Equation 7:
where
Which turns into Equation 8:
Therefore, looking at Figure 28 we quick-select a 47-μH, 2-A inductor (designed for 150-kHz operation) for this application.
This completes the simplified inductor selection procedure. For more general applications and better optimization, see AN-1197 (SNVA038).
10.2.3 Application Curves
(For Continuous Mode Operation)
Figure 28. LM2592HV-3.3
Figure 30. LM2592HV-ADJ
Figure 29. LM2592HV-5.0
