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SLVS897C January 2009 – December 2015 TPS62590

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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 TPS62590 device is a high-efficiency synchronous step-down DC–DC converter featuring power-save mode or 2.25-MHz fixed frequency operation.

8.2 Typical Application

Figure 6. TPS62590DRV Adjustable 1.8 V

8.2.1 Design Requirements

The device operates over an input voltage range from 2.5 V to 5.5 V. The output voltage is adjustable using an external feedback divider.

8.2.2 Detailed Design Procedure

8.2.2.1 Output Voltage Setting

The output voltage can be calculated by Equation 2 with the internal reference voltage V_REF = 0.6 V typically.

Equation 2. TPS62590 q_Vo_Vref_lvs897.gif

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 ~1 MΩ, to keep the network robust against noise. An external feed forward capacitor C1 is required for optimum load transient response. The value of C1 should be in the range between 22 pF and 33 pF.

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

8.2.2.2 Output Filter Design (Inductor and Output Capacitor)

The TPS62590 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 part 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.

8.2.2.2.1 Inductor Selection

The inductor value has a direct effect on the ripple current. The selected inductor has to 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 has also impact on the output voltage ripple in PFM mode. Higher inductor values will lead to lower output voltage ripple and higher PFM frequency, lower inductor values will lead to a higher output voltage ripple but lower PFM frequency.

Equation 3 calculates the maximum inductor current 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 transient the inductor current will rise above the calculated value.

Equation 3. TPS62590 q3_delta_lvs763_.gif

Equation 4. TPS62590 q4_ilmax_lvs763.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 of the corresponding 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 1. List of Inductors

DIMENSIONS [mm³]	INDUCTOR TYPE	SUPPLIER
3 × 3 × 1.5	LPS3015	Coilcraft
3 × 3 × 1.5	LQH3NPN2R2NM0	MURATA
3.2 × 2.6 × 1.2	MIPSA3226D2R2	FDK

8.2.2.2.2 Output Capacitor Selection

The advanced fast-response voltage mode control scheme of the TPS62590 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 over temperature, become resistive at high frequencies.

At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated by Equation 5:

Equation 5. TPS62590 q5_irmsc_lvs763.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 shown in Equation 6:

Equation 6. TPS62590 q6_deltav_lvs763.gif

At light load currents the converter operates in power save mode and the output voltage ripple is dependent 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.

8.2.2.2.3 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 10-μF ceramic capacitor is recommended. 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.

Table 2. List of Capacitor

CAPACITANCE	TYPE	SIZE	SUPPLIER
10 μF	GRM188R60J106M69D	0603 1.6 × 0.8 × 0.8mm³	Murata

Table 3 shows the list of components for the Application Curves.

Table 3. List of Components

COMPONENT REFERENCE	PART NUMBER	MANUFACTURER	VALUE
C_IN	GRM188R60J106M	Murata	10 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