SLVS644E February   2006  – December 2014 TPS61080 , TPS61081

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. 5-V To 12-V, 250-mA Step-Up DC-DC Converter
  5. Revision History
  6. Device Comparison Table
  7. Pin Configuration and Functions
  8. Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Typical Characteristics
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Start Up
      2. 9.3.2 Overcurrent and Short Circuit Protection
      3. 9.3.3 Overvoltage Protection
      4. 9.3.4 Undervoltage Lockout (UVLO)
      5. 9.3.5 Thermal Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 Enable
      2. 9.4.2 Frequency Selection
      3. 9.4.3 Maximum and Minimum Output Current
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Program Output Voltage
      2. 10.1.2 Feed Forward Capacitor
      3. 10.1.3 Soft Start Capacitor
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Inductor Selection
        2. 10.2.2.2 Input And Output Capacitor Selection
      3. 10.2.3 Application Curves
    3. 10.3 System Examples
      1. 10.3.1 Torch Light and Flash Light
      2. 10.3.2 24Vout Output Converter
      3. 10.3.3 30 WLEDs Driver in Media Factor Form Display
        1. 10.3.3.1 ±15 V Dual Output Converter
        2. 10.3.3.2 Step-Up DC-DC Converter with Output Doubler
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Third-Party Products Disclaimer
    2. 13.2 Related Links
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

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

TPS6108x is a highly integrated boost regulator for up to 27-V output with integration of a PWM switch, a power diode as well as an input side isolation switch.TPS6108x adopts current mode control with constant PWM (pulse width modulation) frequency. The switching frequency can be configured to either 600 kHz or 1.2 MHz through the FSW pin.

10.1.1 Program Output Voltage

feedback_lvs644.gifFigure 11. Feed Forward Capacitor Connecting With Feedback Resistor Divider

To program the output voltage, select the values of R1 and R2 (See Figure 11) according to the following equation.

Equation 4. q1_r1_lvs644.gif

A optimum value for R2 is around 50kΩ which sets the current in the resistor divider chain to 1.229 V/50 kΩ = 24.58 μA. The output voltage tolerance depends on the VFB accuracy and the resistor divider.

10.1.2 Feed Forward Capacitor

A feed forward capacitor on the feedback divider, shown in Figure 11, improves transient response and phase margin. This network creates a low frequency zero and high frequency pole at

Equation 5. q9_fz_lvs644.gif
Equation 6. q10_fp_lvs644.gif

The frequency of the pole is determined by C1 and paralleled resistance of R1 and R2. For high output voltage, R1 is much bigger than R2. So

Equation 7. q11_fp_lvs644.gif

The loop gains more phase margin from this network when (Fz+Fp)/2 is placed right at crossover frequency, which is approximately 15 kHz with recommended L and C. The typical value for the zero frequency is between 1 kHz to 10 kHz. For high output voltage, the zero and pole are further apart which makes the feed forward capacitor very effective. For low output voltage, the benefit of the feed forward capacitor is less visible. Table 3 gives the typical R1, R2 and the feed forward capacitor values at the certain output voltage. However, the transient response is not greatly improved which implies that the zero frequency is too high or low to increase the phase margin.

Table 3. Recommended Feed Forward Capacitor Values With Different Output Voltage

Output Voltage R1 R2 C1(Feed Forward)
12V 437kΩ 49.9kΩ 33pF
16V 600kΩ 49.9kΩ 42pF
20V 762kΩ 49.9kΩ 56pF
25V 582kΩ 30.1kΩ 120pF

The 100-Ω resistor is added to reduce noise coupling from the OUT to the FB pin through the feed forward capacitor. Without the resistor, the regulator may oscillate at high output current.

10.1.3 Soft Start Capacitor

The voltage at the SS pin clamps the internal reference voltage, which allows the output voltage to ramp up slowly. The soft start time is calculated as

Equation 8. q6_tss_lvs644.gif

where

Css = soft start capacitor

Iss = soft start bias current (TYP 5 μA)

1.229 V is the typical value of the reference voltage.

During start up, input current has to be supplied to charge the output capacitor. This current is proportional to rising slope of the output voltage, and peaks when output reaches regulation.

Equation 9. q7_iinss_lvs644.gif

Where

Iin_cout = additional input current for charging the output capacitor

The maximum input during soft start is

Equation 10. qnew_iin_lvs644.gif

Output overshoot can occur if the input current at startup exceeds the inductor saturation current and/or reaches current limit because the error amplifier loses control of the voltage feedback loop. The in-rush current can also pulldown input sources, potentially causing system reset. Therefore, select Css to make Iin_ss stay below the inductor saturation current, the IC overcurrent limit and the input's maximum supply current.

TPS6108x can also be configured for constant current output, as shown in the typical applications. In this configuration, a current sense resistor is connected to FB pin for output current regulation. In order to reduce power loss on the sense resistor, FB pin reference voltage can be lowered by connecting a resistor to the SS pin The new reference voltage is simply the resistor value times the SS pin bias current. However, keep in mind that this reference has higher tolerance due to the tolerance of the bias current and sense resistor, and the offset of the clamp circuit. Refer to the specification VCLP and ISS to calculate the tolerance as following.

Equation 11. q8_to_lvs644.gif

Where

Kref = percentage tolerance of the FB reference voltage.
KVclp = percentage tolerance of the clamp circuit.
Klss = percentage tolerance of the SS pin bias current.
KR = percentage tolerance of the SS pin resistor.

Without considering the SS pin resistor tolerance, the FB reference voltage has ±5.6% under the room temperature.

10.2 Typical Application

Figure 12 shows typical application circuit for a step-up DC-DC converter.

typ_app_lvs644.gif Figure 12. 3.3 V to 12 V, 80 mA Step-Up DC-DC Converter

10.2.1 Design Requirements

Table 4. TPS6108x 12-V Output Design Parameters

DESIGN PARAMETERS VALUES
Input Voltage Range 2.5 V to 6 V
Output Voltage 12 V
Transient Response +/- 250 mV
Input Voltage Ripple +/- 50 mV
Output Current 250 mA
Operating Frequency 1.2 MHz

10.2.2 Detailed Design Procedure

10.2.2.1 Inductor Selection

Because the selection of the inductor affects steady state operation, transient behavior and loop stability, the inductor is the most important component in power regulator design. There are three important inductor specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not enough.

The inductance value of the inductor determines the inductor ripple current. It is generally recommended to set peak to peak ripple current given by Equation 4 to 30–40% of DC current. Also, the inductor value should not be beyond the range in the recommended operating conditions table. It is a good compromise of power losses and inductor size. Inductor DC current can be calculated as

Equation 12. q12_ildc_lvs644.gif

The internal loop compensation for PWM control is optimized for the external component shown in the typical application circuit with consideration of component tolerance. Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines saturation current. Using an inductor with a smaller inductance value forces discontinuous PWM in which inductor current ramps down to zero before the end of each switching cycle. It reduces the boost converter’s maximum output current, causes large input voltage ripple and reduces efficiency. An inductor with larger inductance reduces the gain and phase margin of the feedback loop, possibly resulting in instability.

For these reasons, 10μH inductors are recommended for TPS61080 and 4.7μH inductors for TPS61081 for most applications. However, 10μH inductor is also suitable for 600 kHz switching frequency.

Regulator efficiency is dependent on the resistance of its high current path and switching losses associated with the PWM switch and power diode. Although the TPS6108x has optimized the internal switches, the overall efficiency still relies on inductor’s DC resistance (DCR); Lower DCR improves efficiency. However, there is a trade off between DCR and inductor size, and shielded inductors typically have higher DCR than unshielded ones. Table 5 list recommended inductor models.

Table 5. Recommended Inductor For TPS6108x

TPS61080 L
(μH)
DCR MAX
(mΩ)
SATURATION CURRENT
(A)
Size
(L×W×H mm)
VENDOR
VLCF4018T 10 188 0.74 4.0 × 4.0 × 1.8 TDK
CDRH4D16NP 10 118 0.96 4.0 × 4.0 × 1.8 Sumida
LQH43CN100K 10 240 0.65 4.5 × 3.6 × 2.6 Murata
TPS61081 L
(μH)
DCR MAX
(mΩ)
SATURATION CURRENT
(A)
Size
(L×W×H mm)
VENDOR
VLCF5020T 4.7 122 1.74 5.0 × 5.0 × 2.0 TDK
VLCF5014A 6.8 190 1.4 5.0 × 5.0 × 1.4 TDK
CDRH4D14/HP 4.7 140 1.4 4.8 × 4.8 × 1.5 Sumida
CDRH4D22/HP 10 144 1.5 5.0 × 5.0 × 2.4 Sumida

10.2.2.2 Input And Output Capacitor Selection

The output capacitor is mainly selected to meet output ripple and loop stability requirements. This ripple voltage is related to the capacitor’s capacitance and its equivalent series resistance (ESR). Assuming a capacitor with zero ESR, the minimum capacitance needed for a given ripple can be calculated by

Equation 13. q13_cout_lvs644.gif

Vripple = Peak to peak output ripple.

For VIN = 3.6V, VOUT = 20 V, and Fs = 1.2 MHz, 0.1% ripple (20mV) would require 1.0μ capacitor, however, the minimum recommended output capacitor for control loop stability is 4.7 μF. The output capacitor value must be less than 30 µF to ensure the startup current charges the output capacitor to the input voltage in less than 1.7ms. For this value, ceramic capacitors are a good choice for its size, cost and availability.

The additional output ripple component caused by ESR is calculated using:

Equation 14. q14_vrip_lvs644.gif

Due to its low ESR, Vripple_ESR can be neglected for ceramic capacitors, but must be considered if tantalum or electrolytic capacitors are used.

During a load transient, the output capacitor at the output of the boost converter has to supply or absorb transient current before the inductor current ramps up its steady state value. Larger capacitors always help to reduce the voltage over and under shoot during a load transient. A larger capacitor also helps loop stability. Care must be taken when evaluating a ceramic capacitor’s derating under dc bias, aging and AC signal. For example, larger form factor capacitors (in 1206 size) have their self resonant frequencies in the range of the switching frequency. So the effective capacitance is significantly lower. The DC bias can also significantly reduce capacitance. Ceramic capacitors can loss as much as 50% of its capacitance at its rated voltage. Therefore, almost leave margin on voltage rating to ensure adequate capacitance.

See Device Support for popular ceramic capacitor vendors.

10.2.3 Application Curves

sft_st_lvs644.gif
TPS61081
Figure 13. Soft Start
trn_res_lvs644.gif
TPS61080
Figure 15. Transient Response
io_rip_lvs644.gif
TPS61081
Figure 17. Input and Output Ripple
fb_ss_lvs644.gif
TPS61080
Figure 19. SS to FB Accuracy
vo_pro_lvs644.gif
TPS61081
Figure 14. Vout SC Protection
trn2_res_lvs644.gif
TPS61081
Figure 16. Transient Response
ovp_lvs644.gif
TPS61081
Figure 18. OVP
min_ld_lvs644.gif
TPS61081
Figure 20. Minimum Load Requirement

10.3 System Examples

10.3.1 Torch Light and Flash Light

torch_lit_lvs644.gifFigure 21. 50 mA Torch Light and 100 mA Flash Light

10.3.2 24Vout Output Converter

dc2_dc_conv_lvs644.gifFigure 22. 5 V to 24 V, 120 mA Step-Up DC-DC Converter

10.3.3 30 WLEDs Driver in Media Factor Form Display

wled_drv_lvs644.gifFigure 23. 30 WLEDs Driver in Media Factor Form Display

10.3.3.1 ±15 V Dual Output Converter

op_conv_lvs644.gifFigure 24. ±15 V Dual Output Converter

10.3.3.2 Step-Up DC-DC Converter with Output Doubler

stepup_conv_lvs644.gifFigure 25. 5 V to 50 V, 50 mA Step-Up DC-DC Converter with Output Doubler