SLVS957 Data sheet

SLVS957E June 2009 – April 2016 TPS61300 , TPS61301 , TPS61305

PRODUCTION DATA.

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Mechanical Data (Package|Pins)

YFF|20
- MXBG145A

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 TPS6130xx can drive up to three white LEDs in parallel (400-mA, 800-mA, and 400-mA maximum flash current). The extended high-current mode (HC_SEL) allows up to 1025-mA, 2050-mA, and 1025-mA flash current. The 2-MHz switching frequency allows the use of small and low profile passive components.

9.2 Typical Applications

9.2.1 4100-mA Two White High-Power LED Flashlight Featuring Storage Capacitor

TPS61300 TPS61301 TPS61305 TPS61305A TPS61306 storage2_cap_lvs957.gif

Figure 72. 4100-mA Two White High-Power LED Flashlight Featuring Storage Capacitor

9.2.1.1 Design Requirements

For this design example, use the parameters listed in Table 21 as the input parameters.

Table 21. TPS61305 Design Requirement

DESIGN PARAMETER	EXAMPLE VALUE
Input Voltage Range	2.5 V to 5.5 V
Output Voltage	4.95 V
Operating Freqency	2 MHz

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection

A boost converter requires two main passive components for storing energy during the conversion. A boost inductor and a storage capacitor at the output are required. The TPS6130xx device integrates a current limit protection circuitry. The valley current of the PMOS rectifier is sensed to limit the maximum current flowing through the synchronous rectifier and the inductor. The valley peak current limit (250 mA, 500 mA, 1250 mA, or 1750 mA) is user selectable through the I²C interface.

To optimize solution size, the TPS6130xx device has been designed to operate with inductance values between a minimum of 1.3 μH and maximum of 2.9 μH. TI recommends a 2.2-μH inductance in typical high current white LED applications.

The highest peak current through the inductor and the power switch depends on the output load, the input and output voltages. Estimation of the maximum average inductor current and the maximum inductor peak current can be done using Equation 2 and Equation 3:

Equation 2. TPS61300 TPS61301 TPS61305 TPS61305A TPS61306 eq2_ll_lvs957.gif

Equation 3. TPS61300 TPS61301 TPS61305 TPS61305A TPS61306 eq3_llp_lvs957.gif

where

f = switching frequency (2 MHz)
L = inductance value (2.2 μH)
η = estimated efficiency (85%)

The losses in the inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency.

9.2.1.2.2 Input Capacitor

TI recommends low ESR ceramic capacitors for good input voltage filtering. TI recommends a 10-μF input capacitor to improve transient behavior of the regulator and EMI behavior of the total power supply circuit. The input capacitor must be placed as close as possible to the input pin of the converter.

9.2.1.2.3 Output Capacitor

The major parameter necessary to define 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 4:

Equation 4. TPS61300 TPS61301 TPS61305 TPS61305A TPS61306 eq4_cmin_lvs957.gif

where

f is the switching frequency and ΔV is the maximum allowed ripple

With a chosen ripple voltage of 10 mV, a minimum capacitance of 10 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 5:

Equation 5. ΔV_ESR = I_OUT × R_ESR

The total ripple is the sum of the ripple caused by the capacitance and the ripple caused by the ESR of the capacitor. Additional ripple is caused by load transients. This means that the output capacitor has to completely supply the load during the charging phase of the inductor. A reasonable value of the output capacitance depends on the speed of the load transients and the load current during the load change.

For the standard current white LED application (HC_SEL = 0, TPS6130xx), a minimum of 3-μF effective output capacitance is usually required when operating with 2.2-μH (typical) inductors. For solution size reasons, this is usually one or more X5R or X7R ceramic capacitors.

Depending on the material, size and therefore margin to the rated voltage of the used output capacitor, degradation on the effective capacitance can be observed. This loss of capacitance is related to the DC bias voltage applied. TI recommends ensuring the selected capacitors are showing enough effective capacitance under real operating conditions.

To support high-current camera flash application (HC_SEL = 1), the converter is designed to work with a low voltage super-capacitor on the output to take advantage of the benefits they offer. A low-voltage super-capacitor in the 0.1-F to 1.5-F range, and with ESR larger than 40 mΩ, is suitable in the TPS6130xx application circuit. For this device the output capacitor must be connected between the VOUT pin and a good ground connection.

9.2.1.2.4 NTC Selection (TPS61305, TPS61305A, TPS61306)

The TPS61305, TPS61305A, and TPS61306 require a negative thermistor (NTC) for sensing the LED temperature. Once the temperature monitoring feature is activated, a regulated bias current (≈24 μA) will be driven out of the TS port and produce a voltage across the thermistor.

If the temperature of the NTC-thermistor rises due to the heat dissipated by the LED, the voltage on the TS input pin decreases. When this voltage goes below the warning threshold, the LEDWARN bit in REGISTER6 is set. This flag is cleared by reading the register.

If the voltage on the TS input decreases further and falls below hot threshold, the LEDHOT bit in REGISTER6 is set and the device goes automatically in shutdown mode to avoid damaging the LED. This status is latched until the LEDHOT flag gets cleared by software.

The selection of the NTC-thermistor value strongly depends on the power dissipated by the LED and all components surrounding the temperature sensor and on the cooling capabilities of each specific application. With a 220-kΩ (at 25°C) thermistor, the valid temperature window is set between 60°C to 90°C. The temperature window can be enlarged by adding external resistors to the TS pin application circuit. To ensure proper triggering of the LEDWARN and LEDHOT flags in noisy environments, the TS signal may require additional filtering capacitance.

TPS61300 TPS61301 TPS61305 TPS61305A TPS61306 temp_mon2_lvs957.gif

Figure 73. Temperature Monitoring Characteristic

9.2.1.2.5 Checking Loop Stability

The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:

Switching node, SW
Inductor current, I_L
Output ripple voltage, V_OUT(AC)

These are the basic signals that need to be measured when evaluating a switching converter. When the switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations the regulation loop may be unstable. This is often a result of improper board layout or L-C combination.

As a next step in the evaluation of the regulation loop the load transient response needs to be tested. VOUT can be monitored for settling time, overshoot or ringing that helps judge the converter's stability. Without any ringing, the loop has usually more than 45° of phase margin.

Because the damping factor of the circuitry is directly related to several resistive parameters, such as MOSFET r_DS(on), that are temperature dependant, the loop stability analysis has to be done over the input voltage range, output current range, and temperature range.