SNVSA52 Data sheet

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 LM3644 can drive two flash LEDs at currents up to 1.5 A per LED. The 2-MHz/4-MHz DC-DC boost regulator allows for the use of small value discrete external components.

9.2 Typical Application

Figure 41. LM3644 Typical Application

9.2.1 Design Requirements

Example requirements based on default register values:

DESIGN PARAMETER	EXAMPLE VALUE
Input voltage range	2.5 V to 5.5 V
Brightness control	I²C Register
LED configuration	2 parallel flash LEDs
Boost switching frequency	2 MHz (4 MHz selectable)
Flash brightness	750 mA per LED

9.2.2 Detailed Design Procedure

9.2.2.1 Output Capacitor Selection

The LM3644 is designed to operate with a 10-µF ceramic output capacitor. When the boost converter is running, the output capacitor supplies the load current during the boost converter on-time. When the NMOS switch turns off, the inductor energy is discharged through the internal PMOS switch, supplying power to the load and restoring charge to the output capacitor. This causes a sag in the output voltage during the on-time and a rise in the output voltage during the off-time. The output capacitor is therefore chosen to limit the output ripple to an acceptable level depending on load current and input/output voltage differentials and also to ensure the converter remains stable.

Larger capacitors such as a 22-µF or capacitors in parallel can be used if lower output voltage ripple is desired. To estimate the output voltage ripple considering the ripple due to capacitor discharge (ΔV_Q) and the ripple due to the capacitors ESR (ΔV_ESR) use the following equations:

For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:

Equation 1. LM3644 LM3644TT 30171827.gif

The output voltage ripple due to the output capacitors ESR is found by:

Equation 2. LM3644 LM3644TT 30171828.gif

In ceramic capacitors the ESR is very low so the assumption is that 80% of the output voltage ripple is due to capacitor discharge and 20% from ESR. Table 1 lists different manufacturers for various output capacitors and their case sizes suitable for use with the LM3644.

9.2.2.2 Input Capacitor Selection

Choosing the correct size and type of input capacitor helps minimize the voltage ripple caused by the switching of the LM3644 boost converter and reduce noise on the boost converter's input pin that can feed through and disrupt internal analog signals. In the typical application circuit a 10-µF ceramic input capacitor works well. It is important to place the input capacitor as close as possible to the LM3644 input (IN) pin. This reduces the series resistance and inductance that can inject noise into the device due to the input switching currents. Table 1 lists various input capacitors recommended for use with the LM3644.

Table 1. Recommended Input/Output Capacitors (X5R/X7R Dielectric)

MANUFACTURER	PART NUMBER	VALUE	CASE SIZE	VOLTAGE RATING
TDK Corporation	C1608JB0J106M	10 µF	0603 (1.6 mm × 0.8 mm × 0.8 mm)	6.3 V
TDK Corporation	C2012JB1A106M	10 µF	0805 (2.0 mm × 1.25 mm × 1.25 mm)	10 V
Murata	GRM188R60J106M	10 µF	0603 (1.6 mm x 0.8 mm x 0.8 mm)	6.3 V
Murata	GRM21BR61A106KE19	10 µF	0805 (2.0 mm × 1.25 mm × 1.25 mm)	10 V

9.2.2.3 Inductor Selection

The LM3644 is designed to use a 0.47-µH or 1-µH inductor. Table 2 lists various inductors and their manufacturers that work well with the LM3644. When the device is boosting (V_OUT > V_IN) the inductor is typically the largest area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series resistance is important. Additionally, the saturation rating of the inductor should be greater than the maximum operating peak current of the LM3644. This prevents excess efficiency loss that can occur with inductors that operate in saturation. For proper inductor operation and circuit performance, ensure that the inductor saturation and the peak current limit setting of the LM3644 are greater than I_PEAK in the following calculation:

Equation 3. LM3644 LM3644TT 30171829.gif

where

ƒ_SW = 2 or 4 MHz

Efficiency details can be found in the Application Curves.

Table 2. Recommended Inductors

MANUFACTURER	L	PART NUMBER	DIMENSIONS (L×W×H)	I_SAT	R_DC
TOKO	0.47 µH	DFE201610P-R470M	2.0 mm x 1.6 mm x 1.0 mm	4.1 A	32 mΩ
TOKO	1 µH	DFE201610P-1R0M	2.0 mm x 1.6 mm x 1.0 mm	3.7 A	58 mΩ

9.2.3 Application Curves

Ambient temperature is 25°C, input voltage is 3.6 V, HWEN = V_IN, C_IN = 2 × 10 µF, C_OUT = 2 × 10 µF and L = 1 µH, unless otherwise noted.

I_LED = 1.5 A

ƒ_SW = 2 MHz

Flash

Figure 42. 2-MHz LED Efficiency vs Input Voltage

I_LED = 1.5 A	ƒ_SW = 2 MHz	Flash
V_LED = 3.55 V

Figure 44. LED Efficiency vs Input Voltage

I_LED = 1.5 A

ƒ_SW = 2 MHz

Flash

Figure 43. 4-MHz LED Efficiency vs Input Voltage

I_LED = 1.5 A	ƒ_SW = 4 MHz	Flash
V_LED = 3.55 V

Figure 45. LED Efficiency vs Input Voltage

I_LED = 1 A	ƒ_SW = 2 MHz	Flash
V_LED = 3.32 V

Figure 46. LED Efficiency vs Input Voltage

I_{LED1 and LED2} = 729 mA	Flash
V_LED = 3.18 V	ƒ_SW = 2 MHz

Figure 48. LED Efficiency vs Input Voltage

I_LED = 179 mA	ƒ_SW = 4 MHz
V_LED = 2.83 V	Torch

Figure 50. LED Efficiency vs Input Voltage

I_{LED1 and LED2} = 179 mA	ƒ_SW = 4 MHz
V_LED = 2.83 V	Torch

Figure 52. LED Efficiency vs Input Voltage

I_{LED1 and LED2} = 179 mA	ƒ_SW = 4 MHz	LM3644TT
V_LED = 2.83 V	Torch

Figure 54. LED Efficiency vs Input Voltage

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V

Figure 56. Ramp Down

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V

Figure 58. Ripple @ 2 MHz

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V	V_IVFM= 3.2 V

Figure 60. IVFM - Ramp and Hold

	I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
	V_LED = 3.18 V	V_IVFM= 3.2 V

Figure 62. IVFM - Up and Down Adjust

I_LED = 729 mA	ƒ_SW = 2 MHz	Flash
V_LED = 3.18 V

Figure 47. LED Efficiency vs Input Voltage

I_LED = 179 mA	Torch
V_LED = 2.83 V	ƒ_SW = 2 MHz

Figure 49. LED Efficiency vs Input Voltage

I_{LED1 and LED2} = 179 mA	ƒ_SW = 2 MHz
V_LED = 2.83 V	Torch

Figure 51. LED Efficiency vs Input Voltage

I_{LED1 and LED2} = 179 mA	ƒ_SW = 2 MHz	LM3644TT
V_LED = 2.83 V	Torch

Figure 53. LED Efficiency vs Input Voltage

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V

Figure 55. Start-Up

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V

Figure 57. TX Interrupt

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 4 MHz
V_LED = 3.18 V

Figure 59. Ripple @ 4 MHz

I_LED1 = I_LED2 = 730 mA	ƒ_SW = 2 MHz
V_LED = 3.18 V	V_IVFM= 3.2 V

Figure 61. IVFM - Down Adjust Only

9 Applications and Implementation