SNOSB43C September 2011 – November 2016 LM3560
PRODUCTION DATA.
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.
The LM3560 is a synchronous boost flash driver with dual 1000-mA high-side current sources. The 2-MHz DC-DC boost regulator allows for the use of small external components. The device operates from a typical input voltage from 2.5 V to 5.5 V and an ambient temperature range of –40°C to +85°C.
For typical synchronous boost flash driver applications, use the parameters listed in Table 19.
DESIGN PARAMETER | EXAMPLE VALUE |
---|---|
Minimum input voltage | 2.5 V |
Minimum output voltage | 1.8 V |
Maximum output current | 5 V |
Maximum output current | 1.8 A |
Switching frequency | 2 MHz |
The LM3560 is designed to operate with at least a 10-µF ceramic output capacitor. When the boost converter is running the output capacitor supplies the load current during the boost converters 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 NFET on-time and a rise in the output voltage during the NFET off-time. Therefore chose the output capacitor 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.
For proper operation the output capacitor must be at least a 10-µF ceramic. Larger capacitors such as a 22 µF or multiple 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 (ΔVQ) and the ripple due to the equivalent series resistance (ESR) (ΔVESR) of the capacitor use Equation 1 and Equation 2:
For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:
The output voltage ripple due to the output capacitors ESR is found by:
In ceramic capacitors the ESR is very low so a close approximation is to assume that 80% of the output voltage ripple is due to capacitor discharge and 20% from ESR. Table 20 lists different manufacturers for various output capacitors and their case sizes suitable for use with the LM3560.
Choosing the correct size and type of input capacitor helps minimize the input voltage ripple caused by the switching of the LM3560’s boost converter, and reduces noise on the boost converters input terminal that can feed through and disrupt internal analog signals. In Figure 41 a 10-µF ceramic input capacitor works well. It is important to place the input capacitor as close as possible to the input (IN) pins of the LM3560 device. This reduces the series resistance and inductance that can inject noise into the device due to the input switching currents. Table 20 lists various input capacitors that are recommended for use with the LM3560.
MANUFACTURER | PART NUMBER | VALUE | CASE SIZE | VOLTAGE RATING |
---|---|---|---|---|
TDK Corporation | C1608JB0J106M | 10 µF | 0603 (1.6 mm × 0.8mm × 0.8 mm) | 6.3 V |
TDK Corporation | C2012JB1A106M | 10 µF | 0805 (2 mm ×1.25 mm × 1.25 mm) | 10 V |
Murata | GRM21BR61A106KE19 | 10 µF | 0805 (2 mm × 1.25 mm × 1.25 mm) | 10 V |
The LM3560 is designed to use a 1-µH or 2.2-µH inductor. Table 21 lists various inductors and their manufacturers that can work well with the LM3560. When the device is boosting (VOUT > VIN) 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 must be greater than the maximum operating peak current of the LM3560. 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 LM3560 are greater than IPEAK in the following calculation:
MANUFACTURER | L | PART NUMBER | DIMENSIONS (L × W × H) | ISAT | RDC |
---|---|---|---|---|---|
TOKO | 2.2 µH | FDSD0312-H-2R02M | 3 mm × 3.2 mm × 1.2 mm | 2.3 A | 105 mΩ |
TOKO | 1 µH | FDSD0312-H-1R0M | 3 mm × 3.2 mm × 1.2 mm | 3.4 A | 43 mΩ |
TDK | 1 µH | VLS252012T-1R0N | 2 mm × 2.5 mm × 1.2 mm | 2.45 A | 73 mΩ |
TDK | 1 µH | VLS4012ET-1R0N | 4 mm × 4 mm × 1.2 mm | 2.8 A | 50 mΩ |
Programming bit [4] of Configuration register 1 with a 1 selects NTC mode and makes the LEDI/NTC pin a comparator input for flash LED thermal sensing. Figure 42 shows the LM3560 using the NTC thermistor circuit. The thermal sensor resistor divider is composed of R3 and R(T), where R(T) is the negative temperature coefficient thermistor, VBIAS is the bias voltage for the resistive divider, and R3 is used to linearize the response of the NTC around the NTC comparators trip point. CBYP is used to filter noise at the NTC input.
In designing the NTC circuit, values for VBIAS, R(T) and R3, must be chosen. To begin with, NTC thermistors have a non-linear relationship between temperature and resistance:
where
R3 is chosen so that the temperature to resistance relationship becomes more linear and can be found by solving for R3 in the R(T) and R3 resistive divider:
where
As an example, with VBIAS = 2.5 V and a thermistor whose nominal value at 25°C is 100 kΩ and a β = 4500 K, the trip point is chosen to be 93°C. The value of R(T) at 93°C is:
Figure 43 shows the linearity of the thermistor resistive divider of the previous example.
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