SLVS859B June   2008  – December 2014 TPS61085

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Simplified Schematic
  5. 5 Revision History
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Soft-Start
      2. 8.3.2 Frequency Select Pin (FREQ)
      3. 8.3.3 Undervoltage Lockout (UVLO)
      4. 8.3.4 Thermal Shutdown
      5. 8.3.5 Overvoltage Prevention
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Design Procedure
        2. 9.2.2.2 Inductor Selection
        3. 9.2.2.3 Rectifier Diode Selection
        4. 9.2.2.4 Setting the Output Voltage
        5. 9.2.2.5 Compensation (COMP)
        6. 9.2.2.6 Input Capacitor Selection
        7. 9.2.2.7 Output Capacitor Selection
      3. 9.2.3 Application Curves
    3. 9.3 System Examples
      1. 9.3.1 General Boost Application Circuits
      2. 9.3.2 TFT LCD Application Circuit
      3. 9.3.3 WHITE LED Application Circuits
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Trademarks
    2. 12.2 Electrostatic Discharge Caution
    3. 12.3 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9 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.

9.1 Application Information

The TPS61085 is designed for output voltages up to 18.5 V with a switch peak current limit of 2.0 A minimum. The device, which operates in a current mode scheme with quasi-constant frequency, is externally compensated for maximum flexibility and stability. The switching frequency is selectable between 650 kHz and 1.2 MHz, and the input voltage range is 2.3 V to 6.0V. To control the inrush current at start-up a soft-start pin is available. The following section provides a step-by-step design approach for configuring the TPS61085 as a voltage regulating boost converter.

9.2 Typical Application

typ_app2_lvs859.gifFigure 8. Typical Application, 3.3 V to 12 V (fS = 1.2 MHz)

9.2.1 Design Requirements

Table 2. TPS61085 12V Output Design Requirements

PARAMETERS VALUES
Input Voltage 3.3V ± 20%
Output Voltage 12V
Output Current 600mA
Switching Frequency 1.2MHz

9.2.2 Detailed Design Procedure

9.2.2.1 Design Procedure

The first step in the design procedure is to verify that the maximum possible output current of the boost converter supports the specific application requirements. A simple approach is to estimate the converter efficiency, by taking the efficiency numbers from the provided efficiency curves or to use a worst case assumption for the expected efficiency, e.g. 90%.

1. Duty cycle, D:

Equation 1. iq1_lvs821.gif

2. Maximum output current, IOUT(max):

Equation 2. iq2_lvs859.gif

3. Peak switch current in application, ISW(peak):

Equation 3. iq3_lvs859.gif

with the inductor peak-to-peak ripple current, ΔIL

Equation 4. iq4_lvs821.gif

and

VIN Minimum input voltage
VS Output voltage
ILIM(min) Converter switch current limit (minimum switch current limit = 3.2 A)
fS Converter switching frequency (typically 1.2 MHz or 650 kHz)
L Selected inductor value
η Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation)

The peak switch current is the steady state peak switch current that the integrated switch, inductor and external Schottky diode has to be able to handle. The calculation must be done for the minimum input voltage where the peak switch current is the highest.

9.2.2.2 Inductor Selection

The TPS61085 is designed to work with a wide range of inductors. The main parameter for the inductor selection is the saturation current of the inductor which should be higher than the peak switch current as calculated in the Design Procedure section with additional margin to cover for heavy load transients. An alternative, more conservative, is to choose an inductor with a saturation current at least as high as the maximum switch current limit of 3.2 A. The other important parameter is the inductor DC resistance. Usually, the lower the DC resistance the higher the efficiency. It is important to note that the inductor DC resistance is not the only parameter determining the efficiency. Especially for a boost converter where the inductor is the energy storage element, the type and core material of the inductor influences the efficiency as well. At high switching frequencies of 1.2 MHz inductor core losses, proximity effects and skin effects become more important. Usually, an inductor with a larger form factor gives higher efficiency. The efficiency difference between different inductors can vary between 2% to 10%. For the TPS61085, inductor values between 3 μH and 6 μH are a good choice with a switching frequency of 1.2 MHz, typically 3.3 μH. At 650 kHz inductors between 6 μH and 13 μH, typically 6.8 μH are recommended. Possible inductors are shown in Table 3.

Typically, it is recommended that the inductor current ripple is below 35% of the average inductor current. Therefore, the following equation can be used to calculate the inductor value, L:

Equation 5. q1_1_lvs859.gif

with

VIN Minimum input voltage
VS Output voltage
Iout Maximum output current in the application
fS Converter switching frequency (typically 1.2 MHz or 650 kHz)
η Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation)

Table 3. Inductor Selection

L
(μH)		 SUPPLIER COMPONENT CODE SIZE
(L×W×H mm)		 DCR TYP
(mΩ)		 Isat (A)
1.2 MHz
3.3 Sumida CDH38D09 4 x 4 x 1 240 1.25
4.7 Sumida CDPH36D13 5 × 5 × 1.5 155 1.36
3.3 Sumida CDPH4D19F 5.2 x 5.2 x 2 33 1.5
3.3 Sumida CDRH6D12 6.7 x 6.7 x 1.5 62 2.2
4.7 Würth Elektronik 7447785004 5.9 × 6.2 × 3.3 60 2.5
5 Coilcraft MSS7341 7.3 × 7.3 × 4.1 24 2.9
650 kHz
6.8 Sumida CDP14D19 5.2 x 5.2 x 2 50 1
10 Coilcraft LPS4414 4.3 × 4.3 × 1.4 380 1.2
6.8 Sumida CDRH6D12/LD 6.7 x 6.7 x 1.5 95 1.25
10 Sumida CDR6D23 5 × 5 × 2.4 133 1.75
10 Würth Elektronik 744778910 7.3 × 7.3 × 3.2 51 2.2
6.8 Sumida CDRH6D26HP 7 x 7 x 2.8 52 2.9

9.2.2.3 Rectifier Diode Selection

To achieve high efficiency, a Schottky type should be used for the rectifier diode. The reverse voltage rating should be higher than the maximum output voltage of the converter. The averaged rectified forward current Iavg, the Schottky diode needs to be rated for, is equal to the output current IOUT:

Equation 6. iq5_lvs859.gif

Usually a Schottky diode with 2 A maximum average rectified forward current rating is sufficient for most applications. The Schottky rectifier can be selected with lower forward current capability depending on the output current Iout but has to be able to dissipate the power. The dissipated power, PD, is the average rectified forward current times the diode forward voltage, Vforward.

Equation 7. iq5_5_lvs821.gif

Typically the diode should be able to dissipate around 500mW depending on the load current and forward voltage.

Table 4. Rectifier Diode Selection

CURRENT RATING Iavg Vr Vforward / Iavg SUPPLIER COMPONENT CODE PACKAGE TYPE
750 mA 20 V 0.425 V / 750 mA Fairchild Semiconductor FYV0704S SOT 23
1 A 20 V 0.39 V / 1 A NXP PMEG2010AEH SOD 123
1 A 20 V 0.52 V / 1 A Vishay Semiconductor B120 SMA
1 A 20 V 0.5 V / 1 A Vishay Semiconductor SS12 SMA
1 A 20 V 0.44 V / 1 A Vishay Semiconductor MSS1P2L μ-SMP (Low Profile)

9.2.2.4 Setting the Output Voltage

The output voltage is set by an external resistor divider. Typically, a minimum current of 50 μA flowing through the feedback divider gives good accuracy and noise covering. A standard low side resistor of 18 kΩ is typically selected. The resistors are then calculated as:

Equation 8. q3_lvs821.gif

9.2.2.5 Compensation (COMP)

The regulator loop can be compensated by adjusting the external components connected to the COMP pin. The COMP pin is the output of the internal transconductance error amplifier.

Standard values of RCOMP = 13 kΩ and CCOMP = 3.3 nF will work for the majority of the applications.

See Table 5 for dedicated compensation networks giving an improved load transient response. The following equations can be used to calculate RCOMP and CCOMP:

Equation 9. q3_1_lvs859.gif

with

VIN Minimum input voltage
VS Output voltage
Cout Output capacitance
L Inductor value, e.g. 3.3 μH or 6.8 μH
IOUT Maximum output current in the application

Make sure that RCOMP < 120 kΩ and CCOMP> 820 pF, independent of the results of the above formulas.

Table 5. Recommended Compensation Network Values at High/Low Frequency

FREQUENCY L VS VIN ± 20% RCOMP CCOMP Used IOUT
High (1.2 MHz) 3.3 µH 15 V 5 V 82 kΩ 1.1 nF 0.7A
3.3 V 75 kΩ 1.6 nF 0.5A
12 V 5 V 51 kΩ 1.1 nF 0.9A
3.3 V 47 kΩ 1.6 nF 0.6A
9 V 5 V 30 kΩ 1.1 nF 1.2A
3.3 V 27 kΩ 1.6 nF 0.8A
Low (650 kHz) 6.8 µH 15 V 5 V 43 kΩ 2.2 nF 0.7A
3.3 V 39 kΩ 3.3 nF 0.5A
12 V 5 V 27 kΩ 2.2 nF 0.9A
3.3 V 24 kΩ 3.3 nF 0.6A
9 V 5 V 15 kΩ 2.2 nF 1.2A
3.3 V 13 kΩ 3.3 nF 0.8A

Table 5 gives conservative RCOMP and CCOMP values for certain inductors, input and output voltages providing a very stable system. For a faster response time, a higher RCOMP value can be used to enlarge the bandwidth, as well as a slightly lower value of CCOMP to keep enough phase margin. These adjustments should be performed in parallel with the load transient response monitoring of TPS61087.

9.2.2.6 Input Capacitor Selection

For good input voltage filtering low ESR ceramic capacitors are recommended. TPS61085 has an analog input IN. Therefore, a 1 μF bypass is highly recommended as close as possible to the IC from IN to GND.

One 10 μF ceramic input capacitors are sufficient for most of the applications. For better input voltage filtering this value can be increased. Refer to Table 6 and typical applications for input capacitor recommendations.

9.2.2.7 Output Capacitor Selection

For best output voltage filtering a low ESR output capacitor like ceramic capcaitor is recommended. Two 10 μF ceramic output capacitors (or one 22 μF) work for most of the applications. Higher capacitor values can be used to improve the load transient response. Refer to Table 6 for the selection of the output capacitor.

Table 6. Rectifier Input and Output Capacitor Selection

CAPACITOR VOLTAGE RATING SUPPLIER COMPONENT CODE
CIN 10 μF/1206 16 V Taiyo Yuden EMK212 BJ 106KG
IN bypass 1 μF/0603 16 V Taiyo Yuden EMK107 BJ 105KA
COUT 10 μF/1206 25 V Taiyo Yuden TMK316 BJ 106KL

To calculate the output voltage ripple, Equation 10 can be used:

Equation 10. q10_lvs859.gif

with

ΔVC Output voltage ripple dependent on output capacitance,output current and switching frequency
VS Output voltage
VIN Minimum input voltage of boost converter
fS Converter switching frequency (typically 1.2 MHz or 650 kHz)
Iout Output capacitance
ΔVC_ESR Output voltage ripple due to output capacitors ESR (equivalent series resistance)
ISWPEAK Inductor peak switch current in the application
RC_ESR Output capacitors equivalent series resistance (ESR)

ΔVC_ESR can be neglected in many cases since ceramic capacitors provide low ESR.

9.2.3 Application Curves

pwm_wf_lvs859.gifFigure 9. PWM Switching Discontinuous Conduction Mode
ld_res_lvs859.gifFigure 11. Load Transient Response High Frequency (1.2 MHz)
soft_start_lvs859.gifFigure 13. Soft-Start
pwm2_wf_lvs859.gifFigure 10. PWM Switching Continuous Conduction Mode
ld2_res_lvs859.gifFigure 12. Load Transient Response Low Frequency (650 kHz)

9.3 System Examples

9.3.1 General Boost Application Circuits

typ_app3_lvs859.gifFigure 14. Typical Application, 3.3 V to 12 V (fS = 650 kHz)
typ_app4_lvs859.gifFigure 15. Typical Application, 3.3 V to 9 V (fS = 1.2 MHz)
typ_app5_lvs859.gifFigure 16. Typical Application, 3.3 V to 9 V (fS = 650 kHz)
typ_app6_lvs859.gifFigure 17. Typical Application With External Load Disconnect Switch

9.3.2 TFT LCD Application Circuit

typ_app7_lvs859.gifFigure 18. Typical Application 3.3 V to 9 V (fS = 1.2 MHz) for TFT LCD With External Charge Pumps
(VGH, VGL)

9.3.3 WHITE LED Application Circuits

sim_app1_lvs859.gifFigure 19. Simple Application (3.3 V Input - fsw = 650 kHz) for wLED Supply (3S3P)
(With Optional Clamping Zener Diode)
sim_app2_lvs859.gifFigure 20. Simple Application (3.3V Input - fsw = 650 kHz) for wLED Supply (3S3P) With Adjustable Brightness Control Using a PWM Signal on the Enable Pin
(With Optional Clamping Zener Diode)
sim_app3_lvs859.gifFigure 21. Simple Application (3.3 V Input - fsw = 650 kHz) for wLED Supply (3S3P) With Adjustable Brightness Control Using an Analog Signal on the Feedback Pin
(With Optional Clamping Zener Diode)