SNVS974B April 2013 – October 2015 LM3630A
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.
The LM3630A is a dual-channel backlight driver. The device has 5-bit full-scale current programmability (5 mA to 30 mA) and for every full-scale current there is 8 bits of LED current adjustment from 0 to IFULL_SCALE. Both current sinks can be independently controlled via two separate full-scale current registers and two separate 8-bit brightness registers, or can be made to track together via a single brightness register.
For typical white LED applications, use the parameters listed in Table 21.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Minimum input voltage||2.3 V|
|Minimum output voltage||VIN|
|Output current||28.5 mA per channel|
|Switching frequency||500 kHz or 1 MHz|
The LM3630A is designed to work with a 10-µH to 22-µH inductor. When selecting the inductor, ensure that the saturation rating for the inductor is high enough to accommodate the peak inductor current. Equation 4 calculates the peak inductor current based upon LED current, VIN, VOUT, and efficiency.
When choosing L, the inductance value must also be large enough so that the peak inductor current is kept below the LM3630A device's switch current limit. This forces a lower limit on L given by Equation 6.
|MANUFACTURER||PART NUMBER||VALUE||SIZE||CURRENT RATING||DC RESISTANCE|
|TDK||VLF4014ST-100M1R0||10 µH||3.8 mm × 3.6 mm × 1.4 mm||1A||0.22 Ω|
|TDK||VLF302512MT-220M||22 µH||3 mm × 2.5 mm × 1.2 mm||0.43A||0.583 Ω|
The LM3630A device's maximum output power is governed by two factors: the peak current limit (ICL = 1.2 A maximum), and the maximum output voltage (VOVP = 40 V minimum). When the application causes either of these limits to be reached, it is possible that the proper current regulation and matching between LED current strings may not be met.
In the case of a peak current limited situation, when the peak of the inductor current hits the LM3630A device's current limit the NFET switch turns off for the remainder of the switching period. If this happens, each switching cycle the LM3630A begins to regulate the peak of the inductor current instead of the headroom across the current sinks. This can result in the dropout of the feedback-enabled current sinks and the current dropping below its programmed level.
The peak current in a boost converter is dependent on the value of the inductor, total LED current (IOUT), the output voltage (VOUT) (which is the highest voltage LED string + 0.3 V regulated headroom voltage), the input voltage VIN, and the efficiency (Output Power/Input Power). Additionally, the peak current is different depending on whether the inductor current is continuous during the entire switching period (CCM) or discontinuous (DCM) where it goes to 0 before the switching period ends.
For CCM the peak inductor current is given by:
For DCM the peak inductor current is given by:
To determine which mode the circuit is operating in (CCM or DCM), a calculation must be done to test whether the inductor current ripple is less than the anticipated input current (IIN). If ΔIL is < IIN, the device operates in CCM. If ΔIL is > IIN then the device is operating in DCM.
Typically at currents high enough to reach the LM3630A device's peak current limit, the device is operating in CCM.
Application Curves show the output current and output voltage derating for a 10-µH and a 22-µH inductor, at switch frequencies of 500 kHz and 1 MHz. A 10-µH inductor is typically a smaller device with lower on resistance, but the peak currents are higher. A 22-µH inductor provides for lower peak currents, but to match the DC resistance of a 10 µH requires a larger-sized device.
|Frequency = 500 kHz||L = 10 µH|
|Frequency = 500 kHz||L = 22 µH|
|Frequency = 1 MHz||L = 10 µH|
|Frequency = 1 MHz||L = 22 µH|
The recommended initialization sequence for the device registers is as follows: