SNVS459A October   2006  – September 2017 LM2853


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1  Input Capacitor (CIN)
      2. 8.1.2  Soft-Start Capacitor (CSS)
      3. 8.1.3  Soft-Start Capacitor (CSS) and Fault Conditions
      4. 8.1.4  Compensation
      5. 8.1.5  Output Filter Values
      6. 8.1.6  Choosing an Inductance Value
      7. 8.1.7  Output Filter Inductors
      8. 8.1.8  Output Filter Capacitors
      9. 8.1.9  Split-Rail Operation
      10. 8.1.10 Switch Node Protection
    2. 8.2 Typical Application
  9. Layout
    1. 9.1 Layout Guidelines
    2. 9.2 Example Circuit Schematic and Bill of Materials
  10. 10Device and Documentation Support
    1. 10.1 Receiving Notification of Documentation Updates
    2. 10.2 Community Resources
    3. 10.3 Trademarks
    4. 10.4 Electrostatic Discharge Caution
    5. 10.5 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

Application and Implementation


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.

Application Information

Input Capacitor (CIN)

Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying PVIN. The input capacitor, CIN, supplies extra charge when the switcher needs to draw a burst of current from the supply. The RMS current rating and the voltage rating of the CIN capacitor are therefore important in the selection of CIN. The RMS current specification can be approximated by:

Equation 1. LM2853 20201515.gif

where D is the duty cycle, VOUT/VIN. CIN also provides filtering of the supply. Trace resistance and inductance degrade the benefits of the input capacitor, so CIN should be placed very close to PVIN in the layout. A 22 µF or 47 µF ceramic capacitor is typically sufficient for CIN. In parallel with the large input capacitance a smaller capacitor should be added such as a 1 µF ceramic for higher frequency filtering. Ceramic capacitors with high quality dielectrics such as X5R or X7R should be used to provide a constant capacitance across temperature and line variations. For improved load regulation and transient performance, the use of a small 1 µF ceramic capacitor is also recommended as a local bypass for the AVIN pin.

Soft-Start Capacitor (CSS)

The DAC that sets the reference voltage of the error amplifier sources a current through a resistor to set the reference voltage. The reference voltage is one half of the output voltage of the switcher due to the 200 kΩ divider connected to the SNS pin. Upon start-up, the output voltage of the switcher tracks the reference voltage with a two to one ratio as the DAC current charges the capacitance connected to the reference voltage node. Internal capacitance of 20 pF is permanently attached to the reference voltage node which is also connected to the soft start pin, SS. Adding a soft-start capacitor externally increases the time it takes for the output voltage to reach its final level. The charging time required for the reference voltage can be estimated using the RC time constant of the DAC resistor and the capacitance connected to the SS pin. Three RC time constant periods are needed for the reference voltage to reach 95% of its final value. The actual start up time will vary with differences in the DAC resistance and higher-order effects.

If little or no soft-start capacitance is connected, then the start up time may be determined by the time required for the current limit current to charge the output filter capacitance. The capacitor charging equation I = CΔV/Δt can be used to estimate the start-up time in this case. For example, a part with a 3 V output, a 100 μF output capacitance and a 5A current limit threshold would require a time of 60 µs:

Equation 2. LM2853 20201516.gif

Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2853 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In this example, suppose a 3 ms start time is desired. Three time constants are required for charging the soft-start capacitor to 95% of the final reference voltage. So in this case RC = 1 ms. The DAC resistor, R, is 450 kΩ so C can be calculated to be 2.2 nF. A 2.2 nF ceramic capacitor can be chosen to yield approximately a 3 ms start-up time.

Soft-Start Capacitor (CSS) and Fault Conditions

Various fault conditions such as short circuit and UVLO of the LM2853 activate internal circuitry designed to control the voltage on the soft-start capacitor. For example, during a short circuit current limit event, the output voltage typically falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that once the short is removed, the LM2853 can restart gracefully from whatever voltage the output reached during the short circuit event. The range of soft-start capacitors is therefore restricted to values 1 nF to 50 nF.


The LM2853 provides a highly integrated solution to power supply design. The compensation of the LM2853, which is type-three, is included on-chip. The benefit of integrated compensation is straight-forward, simple power supply design. Since the output filter capacitor and inductor values impact the compensation of the control loop, the range of LO, CO and CESR values is restricted in order to ensure stability.

Output Filter Values

Table 1 details the recommended inductor and capacitor ranges for the LM2853 that are suggested for various typical output voltages. Values slightly different than those recommended may be used, however the phase margin of the power supply may be degraded. For best performance when output voltage ripple is a concern, ESR values near the minimum of the recommended range should be paired with capacitance values near the maximum. If a minimum output voltage ripple solution from a 5 V input voltage is desired, a 6.8 μH inductor can be paired with a 220 μF (50 mΩ) capacitor without degraded phase margin.

Table 1. Recommended LO and CO Values

VOUT (V) VIN (V) LO (µH) CO (µF) CESR (mΩ)
0.8 5 4.7 6.8 120 220 70 100
3.3 4.7 4.7 150 220 50 100
1 5 4.7 6.8 120 220 70 100
3.3 4.7 4.7 150 220 50 100
1.2 5 4.7 6.8 120 220 70 100
3.3 4.7 4.7 120 220 60 100
1.5 5 4.7 6.8 120 220 70 100
3.3 4.7 4.7 120 220 60 100
1.8 5 4.7 6.8 120 220 70 120
3.3 4.7 4.7 100 220 70 120
2.5 5 4.7 6.8 120 220 70 150
3.3 4.7 4.7 100 220 80 150
3.0 5 4.7 6.8 120 220 70 150
3.3 4.7 4.7 100 220 80 150
3.3 5 4.7 6.8 120 220 70 150

Choosing an Inductance Value

The current ripple present in the output filter inductor is determined by the input voltage, output voltage, switching frequency and inductance according to Equation 3.

Equation 3. LM2853 20201517.gif

where ΔIL is the peak to peak current ripple, D is the duty cycle VOUT/VIN, VIN is the input voltage applied to the output stage, VOUT is the output voltage of the switcher, f is the switching frequency and LO is the inductance of the output filter inductor. Knowing the current ripple is important for inductor selection since the peak current through the inductor is the load current plus one half the ripple current. Care must be taken to ensure the peak inductor current does not reach a level high enough to trip the current limit circuitry of the LM2853. As an example, consider a 5 V to 1.2 V conversion and a 550 kHz switching frequency. According to Table 1, a 4.7 µH inductor may be used. Calculating the expected peak-to-peak ripple,

Equation 4. LM2853 20201518.gif

The maximum inductor current for a 3A load would therefore be 3A plus 177 mA, 3.177A. As shown in the ripple equation (Equation 4), the current ripple is inversely proportional to inductance.

Output Filter Inductors

Once the inductance value is chosen, the key parameter for selecting the output filter inductor is its saturation current (ISAT) specification. Typically ISAT is given by the manufacturer as the current at which the inductance of the coil falls to a certain percentage of the nominal inductance. The ISAT of an inductor used in an application should be greater than the maximum expected inductor current to avoid saturation. Table 2 lists inductors that are suitable in LM2853 applications.

Table 2. Recommended Inductors

4.7 μF DO3308P-472ML Coilcraft
4.7 μF DO3316P-472ML Coilcraft
4.7 μF MSS1260-472ML Coilcraft
5.2 μF MSS1038-522NL Coilcraft
5.6 μF MSS1260-562ML Coilcraft
6.8 μF DO3316P-682ML Coilcraft
6.8 μF MSS1260-682ML Coilcraft

Output Filter Capacitors

The recommended capacitors that may be used in the output filter with the LM2853 are limited in value and ESR range according to Table 1.

Table 3 shows some examples of capacitors that can typically be used in a LM2853 application.

Table 3. Recommended Capacitors

100 594D107X_010C2T Tantalum Vishay-Sprague
100 593D107X_010D2_E3 Tantalum Vishay-Sprague
100 TPSC107M006#0075 Tantalum AVX
100 NOSD107M006#0080 Niobium Oxide AVX
100 NOSC107M004#0070 Niobium Oxide AVX
120 594D127X_6R3C2T Tantalum Vishay-Sprague
150 594D157X_010C2T Tantalum Vishay-Sprague
150 595D157X_010D2T Tantalum Vishay-Sprague
150 591D157X_6R3C2_20H Tantalum Vishay-Sprague
150 TPSD157M006#0050 Tantalum AVX
150 TPSC157M004#0070 Tantalum AVX
150 NOSD157M006#0070 Niobium Oxide AVX
220 594D227X_6R3D2T Tantalum Vishay-Sprague
220 591D227X_6R3D2_20H Tantalum Vishay-Sprague
220 591D227X_010D2_20H Tantalum Vishay-Sprague
220 593D227X_6R3D2_E3 Tantalum Vishay-Sprague
220 TPSD227M006#0050 Tantalum AVX
220 NOSD227M0040060 Niobium Oxide AVX

Split-Rail Operation

The LM2853 can be powered using two separate voltages for AVIN and PVIN. AVIN is the supply for the control logic; PVIN is the supply for the power FETs. The output filter components need to be chosen based on the value of PVIN. For PVIN levels lower than 3.3 V, use output filter component values recommended for 3.3 V. PVIN must always be equal to or less than AVIN.

LM2853 20201513.gif Figure 9. Split-Rail Operation Example Circuit

Switch Node Protection

The LM2853 includes protection circuitry that monitors the voltage on the switch pin. Under certain fault conditions, switching is disabled in order to protect the switching devices. One side effect of the protection circuitry may be observed when power to the LM2853 is applied with no or light load on the output. The output will regulate to the rated voltage, but no switching may be observed. As soon as the output is loaded, the LM2853 will begin normal switching operation.

Typical Application

LM2853 20201502.gif Figure 10. LM2853 Typical Application Circuit