SLVS762 Data sheet

SLVS762C June 2007 – July 2015 TPS62242

PRODUCTION DATA.

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 TPS6224x device is a high-efficiency synchronous step-down DC-DC converter featuring power save mode or 2.25-MHz fixed-frequency operation.

9.2 Typical Application

TPS62240 TPS62242 TPS62243 adj_12_ai_lvs762.gif

Figure 6. TPS62240DRV Adjustable 1.2 V

9.2.1 Design Requirements

The device operates over an input voltage range from 2 V to 6 V. The output voltage is adjustable using an external feedback divider.

9.2.2 Detailed Design Procedure

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

Table 1. List of Components

COMPONENT REFERENCE	PART NUMBER	MANUFACTURER	VALUE
C_IN	GRM188R60J475K	Murata	4.7 μF, 6.3 V. X5R Ceramic
C_OUT	GRM188R60J106M	Murata	10 μF, 6.3 V. X5R Ceramic
C₁		Murata	22 pF, COG Ceramic
L₁	LPS3015	Coilcraft	2.2 μH, 110 mΩ
R₁, R₂	Values depending on the programmed output voltage

9.2.2.1 Output Voltage Setting

The output voltage can be calculated to:

Equation 2. TPS62240 TPS62242 TPS62243 inl1_vout_lvs762.gif

with an internal reference voltage V_REF typical 0.6 V.

To minimize the current through the feedback divider network, R₂ should be 180 kΩ or 360 kΩ. The sum of R₁ and R₂ should not exceed approximately 1 MΩ, to keep the network robust against noise.

An external feedforward capacitor C₁ is required for optimum load transient response. The value of C₁ should be in the range from 22 pF to 33 pF.

Route the FB line away from noise sources, such as the inductor or the SW line.

9.2.2.2 Output Filter Design (Inductor and Output Capacitor)

The TPS6224x is designed to operate with inductors in the range of 1.5 μH to 4.7 μH and with output capacitors in the range of 4.7 μF to 22 μF. The device is optimized for operation with a 2.2-μH inductor and 10-μF output capacitor.

Larger or smaller inductor values can be used to optimize the performance of the device for specific operation conditions. For stable operation, the L and C values of the output filter may not fall below 1-μH effective inductance and 3.5-μF effective capacitance. Selecting larger capacitors is less critical because the corner frequency of the L-C filter moves to lower frequencies with fewer stability problems.

9.2.2.2.1 Inductor Selection

The inductor value has a direct effect on the ripple current. The selected inductor must be rated for its DC resistance and saturation current. The inductor ripple current (ΔI_L) decreases with higher inductance and increases with higher V_IN or V_OUT.

The inductor selection also has an impact on the output voltage ripple in the PFM mode. Higher inductor values will lead to lower output voltage ripple and higher PFM frequency, and lower inductor values will lead to a higher output voltage ripple but lower PFM frequency.

Equation 3 calculates the maximum inductor current in PWM mode under static load conditions. The saturation current of the inductor should be rated higher than the maximum inductor current as calculated with Equation 4. This is recommended because during heavy load transients the inductor current will rise above the calculated value.

Equation 3. TPS62240 TPS62242 TPS62243 q3_delta_lvs762_.gif

Equation 4. TPS62240 TPS62242 TPS62243 q4_ilmax_lvs762.gif

where

f = Switching frequency (2.25-MHz typical)
L = Inductor value
ΔI_L = Peak-to-peak inductor ripple current
I_Lmax = Maximum inductor current

A more conservative approach is to select the inductor current rating just for the maximum switch current limit I_LIMF of the converter.

Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage ripple, greater core losses, and lower output current capability.

The total losses of the coil have a strong impact on the efficiency of the DC-DC conversion and consist of both the losses in the DC resistance (R_(DC)) and the following frequency-dependent components:

The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
Additional losses in the conductor from the skin effect (current displacement at high frequencies)
Magnetic field losses of the neighboring windings (proximity effect)
Radiation losses

Table 2. List of Inductors

DIMENSIONS (mm³)	INDUCTANCE (μH)	INDUCTOR TYPE	SUPPLIER
2.5 × 2 × 1	2	MIPS2520D2R2	FDK
2.5 × 2 × 1.2	2	MIPSA2520D2R2	FDK
2.5 × 2 × 1	2.2	KSLI-252010AG2R2	Hitachi Metals
2.5 × 2 × 1.2	2.2	LQM2HPN2R2MJ0L	Murata
3 × 3 × 1.4	2.2	LPS3015	Coilcraft

9.2.2.2.2 Output Capacitor Selection

The advanced fast-response voltage mode control scheme of the TPS6224x allows the use of tiny ceramic capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance overtemperature, become resistive at high frequencies.

At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated as:

Equation 5. TPS62240 TPS62242 TPS62243 q5_irmsc_lvs762.gif

At nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor:

Equation 6. TPS62240 TPS62242 TPS62243 q6_deltav_lvs762.gif

At light load currents, the converter operates in power save mode and the output voltage ripple depends on the output capacitor and inductor value. Larger output capacitor and inductor values minimize the voltage ripple in PFM mode and tighten DC output accuracy in PFM mode.

9.2.2.2.3 Input Capacitor Selection

The buck converter has a natural pulsating input current; therefore, a low ESR input capacitor is required for best input voltage filtering, and minimizing the interference with other circuits caused by high input voltage spikes. For most applications, a 4.7-μF to 10-μF ceramic capacitor is recommended. Because ceramic capacitors lose up to 80% of their initial capacitance at 5 V, it is recommended that a 10-μF input capacitor be used for input voltages greater than 4.5 V. The input capacitor can be increased without any limit for better input voltage filtering.

Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input, and the power is being supplied through long wires, such as from a wall adapter, a load step at the output, or V_IN step on the input, can induce ringing at the VIN pin. The ringing can couple to the output and be mistaken as loop instability, or could even damage the part by exceeding the maximum ratings