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ZHCSEV1A February 2016 – March 2016 TPS62770

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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 TPS62770 is a tiny power solution for wearable applications including a 370nA ultra low Iq step-down converter, a slew rate controlled load switch and a dual mode step-up converter. The output voltage of the step-down converter can be selected between 1.0V and 3.0V. The output voltage can be changed during operation. In shutdown mode, the output of the step-down converter is pulled to GND. The integrated load switch is internally connected to the output of the step-down converter and features slew rate control during turn on phase. Once turned off, its output is connected to GND.

The dual mode step-up converter can generate a constant output voltage up to 15V, e.g. for PMOLED supply, or a constant output current, e.g. for LED back light supply. The output voltage can be adjusted up to 15V with external resistors, or set to fixed 12V by connecting the FB pin to VIN. The device features an internal over voltage protection of 17V in case the FB node is left open or tight to GND. It includes an internal rectifier and load disconnect function. When used as constant output current driver, the device offers a PWM to analog converter to scale down the reference voltage according to the duty cycle of the PWM signal.

8.2 Typical Application

8.2.1 System and PMOLED Supply

Figure 9. Step-Up Converter with Adjustable Output Voltage

spacer

Figure 10. Step-Up Converter with Fixed 12-V Output

8.2.1.1 Design Requirements

The device is supplied by an input voltage between 2.5V and 5.5V. In wearable personal electronics this is usually a rechargeable Li-Ion battery / USB port. The step-down converter supplies the system (MCU/BLE radio). In order to supply a PMOLED display, the step-up converter must be configured to operate in constant output voltage mode with BM pin tied to GND before the step-up converter is enabled. Ideally the BM pin is hardwired to GND. The output voltage of the step-up converter is either set by an external resistor divider network (R1/R2), shown in Figure 9. The step-up converter supports an internally fixed 12V output voltage by connecting the FB pin to VIN, shown in Figure 10.

The LOAD output is internally connected to the output of the buck regulator and can supply a sensor or a sub-system, which are temporarily used. In order to achieve better supply voltage decoupling / noise reduction a capacitor can be connected on the LOAD output.

The design guideline provides a component selection to operate the device within the recommended operating conditions. Table 1 shows the components used for the application characteristic curves.

Table 1. Components for Application Characteristic Curves

REFERENCE	DESCRIPTION	VALUE	PACKAGE CODE / SIZE [mm x mm x mm]	MANUFACTURER
CIN	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10 µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT1	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10 µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT2	Ceramic capacitor X5R 25V, GRM188R61E106MA73	10 uF	0603 / 1.6 x 0.8 x 0.8	Murata
L1	Inductor DFE201610C	2.2 µH	2.0 x 1.6 x 1.0	Toko
L2	Inductor VLS302515	10 µH	3.0 x 2.5 x 1.5	TDK

8.2.1.2 Application Curves Step-Down Converter

Figure 11. Efficiency vs. IOUT, VOUT1 = 1.0 V

Figure 13. Efficiency vs. IOUT, VOUT1 = 1.8 V

Figure 15. F_SW vs. IOUT1, VOUT1 = 1.1 V

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_IO_50uA.gif

V_IN = 3.6V		I_OUT = 50 µA
V_OUT = 1.2V

Figure 17. Typical Operation in Power Save Mode

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_IO_50mA.gif

V_IN = 3.6V		I_OUT = 50 mA
V_OUT = 1.2V

Figure 19. Typical Operation in Power Save Mode

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_LT_5mA_200mA.gif

V_IN = 3.6V		I_OUT = 5mA to 200mA
V_OUT = 1.2V	1 µs rise/fall time

Figure 21. Load Transient Performance

TPS62770 SP_TO_buck_VO_1p8V_startup_after_EN.gif

V_IN = 3.6V		I_OUT = 0mA
V_OUT = 1.8V

Figure 23. Startup After EN High

TPS62770 SP_TO_buck_VO_1p8V_VIN_ramp_up_down.gif

V_IN = 0 V to 3.6 V in 100 µs			EN = VIN
V_OUT = 1.8V			I_OUT = 0 mA

Figure 25. VIN Ramp Up/Down

TPS62770 SP_TO_buck_CTRL_VO1p8V_5mA_VIN_3p6V_Load_150R.gif

V_IN = 3.6V		I_OUT1 = 5mA
V_OUT = 1.8V		R_LOAD = 150Ω

Figure 27. Output Load Enable/Disable

Figure 12. Efficiency vs. IOUT, VOUT1 = 1.2 V

Figure 14. Efficiency vs. IOUT, VOUT1 = 3.0 V

Figure 16. VOUT1 = 1.8 V vs IOUT1

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_IO_1mA.gif

V_IN = 3.6V		I_OUT = 1 mA
V_OUT = 1.2V

Figure 18. Typical Operation in Power Save Mode

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_IO_200mA.gif

V_IN = 3.6V		I_OUT = 200 mA
V_OUT = 1.2V

Figure 20. Typical Operation in PWM Mode

TPS62770 SP_TO_buck_VO1p2V_VIN_3p6V_ac_sweep_5mA_200mA.gif

V_IN = 3.6V		I_OUT = 5mA to 200mA
V_OUT = 1.2V	sinusodial I_OUT sweep

Figure 22. AC Load Regulation Performance

TPS62770 SP_TO_buck_VO_1p8V_startup_ramp.gif

V_IN = 3.6V		I_OUT = 0mA
V_OUT = 1.8V	EN altered from low to high

Figure 24. VOUT Ramp Up

TPS62770 SP_TO_buck_VO_1p8V_output_discharge_VO1p8V_no_load.gif

V_IN = 3.6V		I_OUT = 0mA
V_OUT = 1.8V

Figure 26. Output Discharge

8.2.1.3 Application Curves Step-Up Converter Constant 12 V/15 V Output Voltage

Figure 28. Efficiency vs. IOUT, VOUT = 15V

Figure 30. VOUT2 = 12V vs IOUT2

TPS62770 SP_TO_boost_VO12V_VIN_3p6V_2mA.gif

V_IN = 3.6V		I_OUT2 = 2mA
V_OUT = 12V		L = 10µH

Figure 32. Typical Operation PFM Mode

TPS62770 SP_TO_boost_VO12V_VIN_3p6V_ac_20mA.gif

V_IN = 3.6V	I_OUT2 = 0 mA to 20 mA
V_OUT = 12V		L = 10µH

Figure 34. AC Load Regulation Performance

Figure 29. Efficiency vs. IOUT, VOUT = 12V

TA = 25°C	typical switch current lmit I_{LIM_SW}
L = 10µH	I_OUT2 max @ -3% VOUT drop
	COUT2 = 2x 10µF

Figure 31. Maximum Output Current vs VIN for Typical I_LIMSW

TPS62770 SP_TO_boost_VO12V_VIN_3p6V_30mA.gif

V_IN = 3.6V		I_OUT2 = 30mA
V_OUT = 12V		L = 10µH

Figure 33. Typical Operation PWM Mode

TPS62770 SP_TO_boost_startup_1k_load_12V_VO.gif

V_IN = 3.6 V		R_LOAD = 1 kΩ
V_OUT = 12 V		L = 10 µH

Figure 35. Startup after EN High

8.2.2 Step-Up Converter with 5-V Output Voltage

Figure 36. Step-Up Converter Providing 5-V V_OUT2

8.2.2.1 Design Requirements

Table 2. Components for Application Characteristic Curves

REFERENCE	DESCRIPTION	VALUE	PACKAGE CODE / SIZE [mm x mm x mm]	MANUFACTURER
CIN	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10 µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT2 (2x)	Ceramic capacitor X5R 25V, GRM188R61E106MA73	10 uF	0603 / 1.6 x 0.8 x 0.8	Murata
L2	Inductor VLS302515	4.7 µH	3.0 x 2.5 x 1.5	TDK

8.2.2.2 Application Curves

Figure 37. Efficiency vs. IOUT, VOUT = 5.0 V

TPS62770 SP_TO_boost_VO5V_VIN_3p6V_LT_2mA_100mA_L_4p7uH_2x10uF.gif

Figure 38. Transient Response V_OUT2 = 5 V

8.2.3 Step-Up Converter Operating with Constant Output Current

The step-up converter device can be configured to operate as a constant current driver e.g. to power 3 to 4 white LED's in a string. The current through the string is set by the sense resistor R_Sense as shown in Figure 39 To minimize the losses in the sense resistor, the device features a 200mV internal reference. This section describes an application delivering 10mA through an LED string with 4 LED's which is suitable for small display used in wearable applications.

Figure 39. Step-Up Converter with Constant Output Current - Simplified Block Diagram

8.2.3.1 Design Requirements

The Sense resistor to set the maximum output current can be calculated according to Equation 3 The output current IOUT2 can be reduced by applying a PWM signal at pin EN2/PWM according to Equation 4

Equation 3. TPS62770 eq_Rsense_current_mode.gif

Equation 4. TPS62770 eq_IOUT_current_mode_D.gif

Where:

R_Sense = sense resistor in [Ω]

I_OUT2 = output current in [mA]

D_PWM = Dutycycle of the PWM singal at pin EN2/PWM

Table 3. Components for Application Characteristic Curves

REFERENCE	DESCRIPTION	VALUE	PACKAGE CODE / SIZE [mm x mm x mm]	MANUFACTURER
CIN	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10 µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT2	Ceramic capacitor X5R 25V, GRM188R61E106MA73	10 uF	0603 / 1.6 x 0.8 x 0.8	Murata
L2	Inductor VLS302515	10 µH	3.0 x 2.5 x 1.5	TDK
D1-D4	LED LTW-E670DS	n/a		Lite ON

8.2.3.2 Detailed Design Procedure

8.2.3.2.1 Setting The Output Voltage Of The Step-Down Converter

The output voltage is set with the VSEL1-3 pins according to Output Voltage Setting Step-Down Converter. No further external components are required.

8.2.3.2.2 Programming the Output Voltage Of The Step-Up Converter

There are two ways to set the output voltage of the step-up converter. When the FB pin is connected to the input voltage, the output voltage is fixed to 12 V. This function reduces the external components to minimize the solution size. The second way is to use an external resistor divider to set the desired output voltage.

By selecting the external resistor divider R1 and R2, as shown in Equation 5, the output voltage is programmed to the desired value. When the output voltage is regulated, the typical voltage at the FB pin is V_REF of 795 mV.

Equation 5. TPS62770 EQ1_slvscq7.gif

Where:

V_OUT is the desired output voltage

V_REF is the internal reference voltage at the FB pin

8.2.3.2.3 Recommended LC Output Filter

Table 4. Recommended LC Output Filter Combinations for the Step-Down Converter

INDUCTOR VALUE [µH]⁽²⁾	OUTPUT CAPACITOR VALUE [µF]⁽¹⁾
INDUCTOR VALUE [µH]⁽²⁾	10 µF	22 µF
2.2	√⁽³⁾	√

(1) Capacitance tolerance and bias voltage de-rating is anticipated. The effective capacitance can vary by 20% and -50%.

(2) Inductor tolerance and current de-rating is anticipated. The effective inductance can vary by 20% and -30%.

(3) This LC combination is the standard value and recommended for most applications.

Table 5. Recommended LC Output Filter Combinations for Step-Up Converter

INDUCTOR VALUE [µH]⁽²⁾	VOUT	IOUT	OUTPUT CAPACITOR VALUE [µF]⁽¹⁾
INDUCTOR VALUE [µH]⁽²⁾	VOUT	IOUT	10 µF	2 x 10µF
10	9 V –15 V	(IOUT ≤ 30 mA)	√⁽³⁾	√
10	9 V –15 V	(IOUT ≤ 100 mA)		√⁽³⁾
4.7	5 V	(IOUT ≤ 200 mA)		√⁽³⁾

8.2.3.2.4 Inductor Selection Step-Down Converter

The inductor value affects its peak-to-peak ripple current, the PWM-to-PFM transition point, the output voltage ripple and the efficiency. 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 and can be estimated according to Equation 6.

Equation 7 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 7. This is recommended because during a heavy load transient the inductor current rises above the calculated value. A more conservative way is to select the inductor saturation current above the high-side MOSFET switch current limit, I_LIMF.

Equation 6. TPS62770 eq4_dil_lvs941.gif

Equation 7. TPS62770 eq5_ilmax_lvs941.gif

With:

f = Switching Frequency
L = Inductor Value
ΔI_L= Peak to Peak inductor ripple current
I_Lmax = Maximum Inductor current

In DC/DC converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e. quality factor) and by the inductor DCR value. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current.

The total losses of the coil 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

8.2.3.2.5 Inductor Selection Step-Up Converter

The step-up converter is optimized to work with an inductor values of 10 µH. Follow Equation 8 to Equation 10 to calculate the inductor’s peak current for the application. To calculate the current in the worst case, use the minimum input voltage, maximum output voltage, and maximum load current of the application. To have enough design margin, choose the inductor value with -30% tolerance, and a low power-conversion efficiency for the calculation.

In a step-up regulator, the inductor dc current can be calculated with Equation 8.

Equation 8. TPS62770 EQ2_slvscq7.gif

Where:

V_OUT = output voltage

I_OUT = output current

V_IN = input voltage

η = power conversion efficiency, use 80% for most applications

The inductor ripple current is calculated with the Equation 9 for an asynchronous step-up converter in continuous conduction mode (CCM).

Equation 9. TPS62770 EQ3_slvscq7.gif

Where:

ΔI_L(P-P) = inductor ripple current

L = inductor value

f _SW = switching frequency

V_OUT = output voltage

V_IN = input voltage

Therefore, the inductor peak current is calculated with Equation 10.

Equation 10. TPS62770 EQ4_slvscq7.gif

The following inductor series from different suppliers have been used:

Table 6. List Of Inductors

CONVERTER	INDUCTANCE [µH]	DIMENSIONS [mm³]	INDUCTOR TYPE	SUPPLIER⁽¹⁾
Step-down	2.2	2.9 x 1.6 x 1.0	DFE201610C	TOKO
	2.2	2.0 × 1.25 × 1.0	MIPSZ2012D 2R2	FDK
	2.2	2.0 x 1.2 x 1.0	MDT2012CH2R2	TOKO
Step-up	10	2.0x1.6x1.2	VLS201610	TDK
	10	3.0 x 2.5 x 1.5	VLS302515	TDK
	4.7	3.0 x 2.5 x 1.5	VLS302515	TDK
	4.7	2.0 x 1.6 x 1.5	VLS201612	TDK

(1) See Third-party Products Disclaimer

8.2.3.2.6 DC/DC Input and Output Capacitor Selection

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 light load currents, the converter operates in Power Save Mode and the output voltage ripple is dependent on the output capacitor value and the PFM peak inductor current. A 10 µF ceramic capacitor is recommended as input capacitor.

Table 7 shows a list of tested input/output capacitors.

Table 7. List Of Capacitors

REFERENCE	DESCRIPTION	VALUE	PACKAGE CODE / SIZE [mm x mm x mm]	MANUFACTURER⁽¹⁾
CIN	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT1	Ceramic capacitor X5R 6.3V, GRM155R60J106ME11	10µF	0402 / 1.0 x 0.5 x 0.5	Murata
COUT2	Ceramic capacitor X5R 25V, GRM188R61E106MA73	10uF	0603 / 1.6 x 0.8 x 0.8	Murata
COUT2	Ceramic capacitor X5R 6.3V, GRM188R60J106ME84	10uF	0603 / 1.6 x 0.8 x 0.8	Murata

(1) See Third-party Products Disclaimer

8.2.3.3 Application Curves

TPS62770 SP_TO_boost_LED4_10mA_100perc_T150us.gif

V_IN = 3.6V			R_Sense= 20Ω
EN2/PWM = high			4 LEDs in series
D = 100%, I_LED = 10mA			L = 10µH

Figure 40. Constant Current Operation with EN2/PWM = 100% D

TPS62770 SP_TO_boost_LED4_1mA_10perc_T150us.gif

V_IN = 3.6V		R_Sense= 20 Ω
t_{Dim_On} = 15 µs, t_{Dim_Off} = 135 µs		4 LED's in series
D = 10%, T_DIim = 140 µs, I_LED = 1 mA		L = 10 µH

Figure 42. Constant Current with EN2/PWM = 10% D

TPS62770 SP_TO_boost_LED4_5mA_50perc_T150us.gif

V_IN = 3.6V		R_Sense= 20Ω
t_{Dim_On} = 75 µs, t_{Dim_Off} = 75µs		4 LED's in series
D = 50%, T_DIim = 140µs, I_LED = 5mA		L = 10µH

Figure 41. Constant Current with EN2/PWM = 50% D

V_IN = 3.6V		R_Sense= 20 Ω
T_A = 25°C		LED's in string configuration
T_DIim = 50 µs (F = 20kHz)		L = 10 µH

Figure 43. Constant Current vs D