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ZHCSDN2 April 2015 LM3632A

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- MXBG359A

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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 LM3632A integrates an LCD backlight driver, LCM positive and negative bias voltage supplies, and a flash driver into a single device. The backlight boost converter generates the high voltage required for the LEDs. The device can drive one or two LED strings with 4 to 8 white LEDs per string. Positive and negative bias voltages are post-regulated from the LCM bias boost output voltage. The flash driver can supply constant current of up to a 1.5 A to the LED output. All three functions are independent of each other and can be enabled using their own dedicated controls.

8.2 Typical Application

Figure 52. Typical Application Schematic

8.2.1 Design Requirements

Example requirements are shown below:

DESIGN PARAMETER	EXAMPLE VALUE
Input voltage range	2.7 V to 4.5 V (single Li-Ion cell battery)
Brightness control	I²C Register
Backlight LED configuration	2 parallel, 6 series
Backlight LED current	max 25 mA / string
Backlight boost maximum voltage	29 V
Backlight boost SW frequency	1 MHz
Backlight boost inductor	10 µH, 1-A saturation current
LCM boost output voltage	6 V
V_VNEG output voltage	–5.4 V
V_VPOS output voltage	5.5 V
Flash LED current	1.5 A
Torch LED current	100 mA

8.2.2 Detailed Design Procedure

8.2.2.1 External Components

Table 23 shows examples of external components for the LM3632A. Boost converter output capacitors can be replaced with dual output capacitors of lower capacitance as long as the minimum effective capacitance requirement is met. DC bias effect of the ceramic capacitors must be taken into consideration when choosing the output capacitors. This is especially true for the high output-voltage backlight-boost converter.

Table 23. Recommended External Components

DESIGNATOR (Figure 52)	DESCRIPTION	VALUE	EXAMPLE
C1, C3, C4, C5, C6	Ceramic capacitor	10 µF, 10 V	C1608X5R0J106M
C2	Ceramic capacitor	1 µF, 35 V	C2012X7R1H105K125AB
L1	Inductor	10 µH, 1 A	VLF403212MT- 100M
L2	Inductor	1 µH, 2.8 A	DFE201610P-1R0M
L3	Inductor	2.2 µH, 1 A	VLS201612ET-2R2M
D1	Schottky diode	30 V, 500 mA	NSR0530P2T5G

8.2.2.2 Inductor Selection

Both of the LM3632A boost converters are internally compensated. The compensation parameters are designed for the inductance values listed on Table 23. Effective inductance of the inductors should be ±20%.

There are two main considerations when choosing an inductor: the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of the application should be requested from the manufacturer. The saturation current should be greater than the sum of the maximum load current and the worst-case average-to-peak inductor current. When the boost device is boosting (V_OUT > V_IN) the inductor is one of the largest area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series resistance is important, especially for the flash and LCM Bias converters. For proper inductor operation and circuit performance, ensure that the inductor saturation and the peak current limit setting of the LM3632A are greater than I_PEAK in Equation 5:

Equation 5. LM3632A IND_IPEAK.gif

See detailed information in “Understanding Boost Power Stages in Switch Mode Power Supplies” http://focus.ti.com/lit/an/slva061/slva061.pdf. “Power Stage Designer™ Tools” can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

8.2.2.3 Boost Output Capacitor Selection

At least an 1-μF capacitor is recommended for the backlight boost converter output capacitor. A high-quality ceramic type X5R or X7R is recommended. Voltage rating must be greater than the maximum output voltage that is used. The effective output capacitance should always remain higher than 0.4 µF for stable operation.

For the LCM bias boost output a high-quality 10-μF ceramic type X5R or X7R capacitor is recommended. Voltage rating must be greater than the maximum output voltage that is used.

The flash driver 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's 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.

The DC-bias effect of the capacitors must be taken into consideration when selecting the output capacitors. The effective capacitance of a ceramic capacitor can drop down to less than 10% with maximum rated DC bias voltage. Note that with a same voltage applied, the capacitors with higher voltage rating suffer less from the DC-bias effect than capacitors with lower voltage rating.

8.2.2.4 Input Capacitor Selection

Choosing the correct size and type of input capacitor helps minimize the voltage ripple caused by the switching of the LM3632A boost converters and reduce noise on the boost converter's input pin that can feed through and disrupt internal analog signals. In Figure 52 a 10-μF ceramic input capacitor works well. It is important to place the input capacitor as close as possible to the LM3632A input (VIN) pin. This reduces the series resistance and inductance that can inject noise into the device due to the input switching currents.

8.2.3 Application Curves

8.2.3.1 Backlight Curves

Ambient temperature is 25°C and V_IN is 3.7 V unless otherwise noted. Backlight System Efficiency is defined as PLED / PIN, where PLED is actual power consumed in backlight LEDs.

2p7s LEDs

ƒ = 500 kHz

Figure 53. Backlight Boost Efficiency

2p7s LEDs

ƒ = 1 MHz

Figure 55. Backlight Boost Efficiency

2p7s LEDs

ƒ = 500 kHz

Figure 57. Backlight Boost Efficiency

2p7s LEDs

ƒ = 1 MHz

Figure 59. Backlight Boost Efficiency

2p6s LEDs

ƒ = 500 kHz

Figure 61. Backlight Boost Efficiency

2p6s LEDs

ƒ = 1 MHz

Figure 63. Backlight Boost Efficiency

2p6s LEDs

ƒ = 500 kHz

Figure 65. Backlight Boost Efficiency

2p6s LEDs

ƒ = 1 MHz

Figure 67. Backlight Boost Efficiency

2p7s LEDs

ƒ = 500 kHz

Figure 54. Backlight System Efficiency

2p7s LEDs

ƒ = 1 MHz

Figure 56. Backlight System Efficiency

2p7s LEDs

ƒ = 500 kHz

Figure 58. Backlight System Efficiency

2p7s LEDs

ƒ = 1 MHz

Figure 60. Backlight System Efficiency

2p6s LEDs

ƒ = 500 kHz

Figure 62. Backlight System Efficiency

2p6s LEDs

ƒ = 1 MHz

Figure 64. Backlight System Efficiency

2p6s LEDs

ƒ = 500 kHz

Figure 66. Backlight System Efficiency

2p6s LEDs

ƒ = 1 MHz

Figure 68. Backlight System Efficiency

8.2.3.2 LCM Bias Curves

Ambient temperature is 25°C and V_IN is 3.7 V unless otherwise noted. VPOS, VNEG and VPOS/VNEG Efficiency is defined as POUT / PIN, where POUT is actual power consumed in VPOS, VNEG and (VPOS + VNEG) outputs, respectively.

V_{LCM_OUT} = 4.5 V

Figure 69. LCM Boost Efficiency

V_{LCM_OUT} = 5 V

Figure 70. LCM Boost Efficiency

V_{LCM_OUT} = 5.5 V

Figure 71. LCM Boost Efficiency

V_{LCM_OUT} = 4.8 V

Figure 73. LCM Boost Efficiency

V_{LCM_OUT} = 5.9 V

Figure 75. LCM Boost Efficiency

V_VPOS = 5 V

V_{LCM_OUT} = 5.4 V

Figure 77. VPOS Efficiency

V_VPOS = 4.5 V	V_{LCM_OUT} = 4.9 V

Figure 79. VPOS Efficiency

V_VPOS = 5.5 V

V_{LCM_OUT} = 5.9 V

Figure 81. VPOS Efficiency

V_VNEG = –5 V

V_{LCM_OUT} = 5.4 V

Figure 83. VNEG Efficiency

V_VNEG = –4.5 V

V_{LCM_OUT} = 4.9 V

Figure 85. VNEG Efficiency

V_VNEG = –5.5 V

V_{LCM_OUT} = 5.9 V

Figure 87. VNEG Efficiency

V_VPOS = 5 V

V_VNEG = –5 V

V_{LCM_OUT} = 5.4 V

Figure 89. VPOS/VNEG Efficiency

V_VPOS = 4.5 V

V_VNEG = –4.5 V

V_{LCM_OUT} = 4.9 V

Figure 91. VPOS/VNEG Efficiency

V_VPOS = 5.5 V

V_VNEG = –5.5 V

V_{LCM_OUT} = 5.9 V

Figure 93. VPOS/VNEG Efficiency

V_{LCM_OUT} = 5.5 V

Figure 95. V_{LCM_OUT} Load Regulation

V_VPOS = 4.5 V

V_{LCM_OUT} = 4.9 V

Figure 97. V_VPOS Load Regulation

V_VPOS = 5.5 V

V_{LCM_OUT} = 5.9 V

Figure 99. V_VPOS Load Regulation

V_VNEG = –5 V

V_{LCM_OUT} = 5.4 V

Figure 101. V_VNEG Load Regulation

V_{LCM_OUT} = 6 V

Figure 72. LCM Boost Efficiency

V_{LCM_OUT} = 5.3 V

Figure 74. LCM Boost Efficiency

V_VPOS = 4.5 V

V_{LCM_OUT} = 4.9 V

Figure 76. VPOS Efficiency

V_VPOS = 5.5 V	V_{LCM_OUT} = 5.9 V

Figure 78. VPOS Efficiency

V_VPOS = 5 V

V_{LCM_OUT} = 5.4 V

Figure 80. VPOS Efficiency

V_VNEG = –4.5 V

V_{LCM_OUT} = 4.9 V

Figure 82. VNEG Efficiency

V_VNEG = –5.5 V	V_{LCM_OUT} = 5.9 V

Figure 84. VNEG Efficiency

V_VNEG = –5 V

V_{LCM_OUT} = 5.4 V

Figure 86. VNEG Efficiency

V_VPOS = 4.5 V

V_VNEG = –4.5 V

V_{LCM_OUT} = 4.9 V

Figure 88. VPOS/VNEG Efficiency

V_VPOS= 5.5 V

V_VNEG = –5.5 V

V_{LCM_OUT} = 5.9 V

Figure 90. VPOS/VNEG Efficiency

V_VPOS = 5 V

V_VNEG = –5 V

V_{LCM_OUT} = 5.4 V

Figure 92. VPOS/VNEG Efficiency

V_{LCM_OUT} = 5 V

Figure 94. V_{LCM_OUT} Load Regulation

V_{LCM_OUT} = 6 V

Figure 96. V_{LCM_OUT} Load Regulation

V_VPOS = 5 V

V_{LCM_OUT} = 5.4 V

Figure 98. V_VPOS Load Regulation

V_VNEG = –4.5 V

V_{LCM_OUT} = 4.9 V

Figure 100. V_VNEG Load Regulation

V_VNEG = –5.5 V

V_{LCM_OUT} = 5.9 V

Figure 102. V_VNEG Load Regulation

8.2.3.3 Flash Curves

Ambient temperature is 25°C and V_IN is 3.7 V unless otherwise noted. Flash System Efficiency defined as PLED / PIN, where PLED is actual power consumed in flash LED.