SLVS530D SEPTEMBER   2005  – October 2015 TPS63700

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 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. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Enable
      2. 7.3.2 Load Disconnect
      3. 7.3.3 Output Overvoltage Protection
      4. 7.3.4 Undervoltage Lockout
      5. 7.3.5 Overtemperature Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Soft-Start
      2. 7.4.2 PWM Operation
      3. 7.4.3 Power Save Mode Operation
      4. 7.4.4 Control
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Programming the Output Voltage: Converter
          1. 8.2.2.1.1 Inductor Selection
        2. 8.2.2.2 Capacitor Selection
          1. 8.2.2.2.1 Input Capacitor
          2. 8.2.2.2.2 Output Capacitors
        3. 8.2.2.3 Stabilizing the Control Loop
          1. 8.2.2.3.1 Feedback Divider
          2. 8.2.2.3.2 Compensation Capacitor
      3. 8.2.3 Application Curves
    3. 8.3 System Example
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8 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.

8.1 Application Information

The TPS63700 DC/DC converter is intended for systems typically powered by a single-cell Li-ion or Li-polymer battery with a terminal voltage between 2.7 V up to 4.2 V. Due to the recommended input voltage going up to 5.5 V, the device is also suitable for 3-cell alkaline, NiCd,or NiMH batteries, as well as regulated supply voltages of 3.3 V or 5 V.

8.2 Typical Application

TPS63700 ai_5vout_lvs530.gif Figure 2. Circuit for –5-V Output

8.2.1 Design Requirements

The design of the inverter can be adapted to different output voltage and load current needs by choosing external components appropriately. The following design procedure is adequate for the whole VIN, VOUT and load current range of TPS63700.

Table 1 shows the list of components for the Application Curves.

Table 1. List of Components

REFERENCE DESCRIPTION
C1, C2, C3, C4, X7R/X5R ceramic
C5 4 × 4.7 μF X7R/X5R ceramic
D1 SL03/SL02 Vishay
L1 –5V: TDK VLF4012 4R7, TDK SLF6025-4R7, Coilcraft LPS4018-472,
–12V: Sumida CDRH5D18 10 μH

8.2.2 Detailed Design Procedure

8.2.2.1 Programming the Output Voltage: Converter

The output voltage of the TPS63700 converter can be adjusted with an external resistor divider connected to the FB pin. The reference point of the feedback divider is the reference voltage VREF with 1.213 V. The typical value of the voltage at the FB pin is 0 V. The minimum recommended output voltage at the converter is –15 V. The feedback divider current should be 10 μA. The voltage across R2 is 1.213 V. Based on those values, the recommended value for R2 should be 120 kΩ to 200 kΩ in order to set the divider current at the required value. The value of the resistor R3 can then be calculated using Equation 1, depending on the needed output voltage (VOUT).

Equation 1. TPS63700 Q2_lvs530.gif

For example, if an output voltage of –5 V is needed and a resistor of 150 kΩ has been chosen for R2, a 619-kΩ resistor is needed to program the desired output voltage.

8.2.2.1.1 Inductor Selection

An inductive converter normally requires two main passive components for storing energy during the conversion. An inductor and a storage capacitor at the output are required.

The average inductor current depends on the output load, the input voltage VIN, and the output voltage VOUT. It can be estimated with Equation 2, which shows the formula for the inverting converter.

Equation 2. TPS63700 q2_ilmean_lvs530.gif

where

  • ILavg= Average inductor current

An important parameter for choosing the inductor is the desired current ripple in the inductor.

A ripple current value between 20% and 80% of the average inductor current can be considered as reasonable, depending on the application requirements. A smaller ripple reduces the losses in the inductor, as well as output voltage ripple and EMI. But in the same way, the inductor becomes larger and more expensive.

Keeping those parameters in mind, the possible inductor value can be calculated using Equation 3.

Equation 3. TPS63700 q3_l_lvs530.gif

where

  • ΔIL = Peak-to-peak ripple current
  • f = Switching frequency
  • L = Inductor value

With the known inductor current ripple, the peak inductor value can be approximated with Equation 4. The peak current through the switch and the inductor depends also on the output load, the input voltage VIN, and the output voltage VOUT. To select the right inductor, it is recommended to keep the possible peak inductor current below the current-limit threshold of the power switch. For example, the current-limit threshold of the TPS63700 switch for the inverting converter is nominally 1000 mA.

Equation 4. TPS63700 q4_ilmax4_lvs530.gif

where

  • ILMAX = Peak inductor current
  • ΔIL = Peak-to-peak ripple current

With Equation 5, the inductor current ripple at a given inductor can be approximated.

Equation 5. TPS63700 q5_delta_lvs530.gif

where

  • ΔIL = Peak-to-peak ripple current
  • L = Inductor value
  • f = Switching frequency

Care has to be taken for the possibility that load transients and losses in the circuit can lead to higher currents as estimated in Equation 4. Also, the losses caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency.

The following inductor series from different suppliers have been tested with the TPS63700 converter, see Table 2.

Table 2. List of Inductors

Output Voltage Vendor SUGGESTED INDUCTOR
–5 V TDK VLF4012 4.7 μH
SLF6025-4.7 μH
–5 V Coilcraft LPS4018 4.7 μH
LPS3015 4.7 μH
–12 V Sumida CDRH5D18 10 μH
–12 V Coilcraft MOS6020 10 μH

8.2.2.2 Capacitor Selection

8.2.2.2.1 Input Capacitor

At least a 10-μF ceramic input capacitor is recommended for a good transient behavior of the regulator, and EMI behavior of the total power supply circuit.

8.2.2.2.2 Output Capacitors

One of the major parameters necessary to define the capacitance value of the output capacitor is the maximum allowed output voltage ripple of the converter. This ripple is determined by two parameters of the capacitor, the capacitance and the ESR. It is possible to calculate the minimum capacitance needed for the defined ripple, supposing that the ESR is zero, by using Equation 6 for the inverting converter output capacitor.

Equation 6. TPS63700 Q8_lvs530.gif

where

  • f = Switching frequency
  • ΔV = Maximum allowed ripple
  • Cmin = Minimum capacitance

With a chosen ripple voltage in the range of 10 mV, a minimum capacitance of 12 μF is needed. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 7 .

Equation 7. TPS63700 Q10_lvs530.gif

where

  • ΔVESR = Voltage ripple caused by RESR of capacitor
  • RESR = Equivalent series resistance of capacitor

An additional ripple of 2 mV is the result of using a typical ceramic capacitor with an ESR in a 10-mΩ range. The total ripple is the sum of the ripple caused by the capacitance, and the ripple caused by the ESR of the capacitor. In this example, the total ripple is 12 mV. Additional ripple is caused by load transients. When the load current increases rapidly, the output capacitor must provide the additional current until the inductor current has been increased by the control loop by setting a higher on-time at the main switch (duty cycle). The higher duty cycle results in longer inductor charging periods, but the rate of increase of the inductor current is also limited by the inductance itself. When the load current decreases rapidly, the output capacitor needs to store the excessive energy (stored in the inductor) until the regulator has decreased the inductor current by reducing the duty cycle. The recommendation is to use higher capacitance values, as the previous calculations show.

8.2.2.3 Stabilizing the Control Loop

8.2.2.3.1 Feedback Divider

To speed up the control loop, a feed-forward capacitor of 10 pF is recommended in the feedback divider, parallel to R3.

To avoid coupling noise into the control loop from the feed-forward capacitor, the feed-forward effect can be bandwidth-limited by adding series resistor R4. A value in the range of 100 kΩ is suitable. The higher the resistance, the lower the noise coupled into the control loop system.

8.2.2.3.2 Compensation Capacitor

The control loop of the converter is completely compensated internally. However the internal feed-forward system requires an external capacitor. A 4.7-nF capacitor at the COMP pin of the converter is recommended.

8.2.3 Application Curves

TPS63700 eff_oi1_lvs530.gif Figure 3. Efficiency vs Output Current,
VOUT –5 V
TPS63700 eff_oi3_lvs530.gif Figure 5. Efficiency vs Output Current,
VOUT –15 V
TPS63700 eff_vo2psm_lvs530.gif Figure 7. Efficiency vs Input Voltage,
VOUT –12 V
TPS63700 vo_io2_lvs530.gif Figure 9. Output Voltage vs Output Current
TPS63700 vo_pwm_lvs530.gif Figure 11. Output Voltage in
Continuous Conduction Mode
TPS63700 line_tr_lvs530.gif Figure 13. Line Transient Response, –5 V
TPS63700 eff_oi2_lvs530.gif Figure 4. Efficiency vs Output Current,
VOUT –12 V
TPS63700 eff_vo1psm_lvs530.gif Figure 6. Efficiency vs Input Voltage,
VOUT –5 V
TPS63700 vo_io1_lvs530.gif Figure 8. Output Voltage vs Output Current
TPS63700 vo2_pwm_lvs530.gif Figure 10. Output Voltage in
Discontinuous Conduction Mode
TPS63700 ld_trans_lvs530.gif Figure 12. Load Transient Response,
–5 V, 45 to 150 mA
TPS63700 start_up_lvs530.gif Figure 14. Start-Up After Enable, –5 V

8.3 System Example

TPS63700 ai_12vout_lvs530.gif Figure 15. Circuit for –12-V Output