Datasheet LT8410, LT8410-1 (Analog Devices) - 9

LT8410/LT8410-1
APPLICATIONS INFORMATION
or 0805 size capacitor will be adequate. A 0.1μF to 1μF
Connecting the Load to the CAP Node
capacitor placed on the CAP node is recommended to filter The efficiency of the converter can be improved by con- the inductor current, while a 0.1μF to 1μF capacitor placed necting the load to the CAP pin instead of the V on the V OUT pin. OUT node will give excellent transient response and stability. To make the VREF pin less sensitive to noise, The power loss in the PMOS disconnect circuit is then putting a capacitor on the VREF pin is recommended, but not made negligible. No quiescent current will be consumed required. A 47nF to 220nF 0402 capacitor will be sufficient. in the internal feedback resistor divider string during Table 2 shows a list of several capacitor manufacturers. shutdown since the PMOS transistor will be open and the Consult the manufacturers for more detailed information internal feedback resistor divider is connected at the VOUT and for their entire selection of related parts. pin. The disadvantage of this method is that the CAP node
Table 2. Recommended Ceramic Capacitor Manufacturers
cannot go to ground during shutdown, but will be limited to around a diode drop below V
MANUFACTURER PHONE WEB SITE
CC. Loads connected to the part should only sink current. Never force external power Taiyo Yuden (408) 573-4150 www.t-yuden.com supplies onto the CAP or V Murata (814) 237-1431 www.murata.com OUT pins. AVX (843) 448-9411 www.avxcorp.com
Maximum Output Load Current
Kemet (408) 986-0424 www.kemet.com The maximum output current of a particular LT8410 series TDK (847) 803-6100 www.tdk.com circuit is a function of several circuit variables. The following
Setting Output Voltage
method can be helpful in predicting the maximum load current for a given circuit: The output voltage is set by the FBP pin voltage. VOUT is equal to 31.85 • V Step 1. Calculate the peak inductor current: FBP when the output is regulated, as shown in Figure 1. Since the VREF pin provides a good V reference (1.235V), the FBP voltage can be easily set by I IN • 150 • 10–6 PK = ILIMIT + mA L a resistor divider from the V REF pin to ground. The series resistance of this resistor divider should be kept larger than where ILIMIT is 25mA and 8mA for LT8410 and LT8410-1 200KΩ to prevent loading down the VREF pin. The FBP pin respectively. L is the inductance value in henrys and VIN can also be biased directly by an external reference. For is the input voltage to the boost circuit. overvoltage protection, the output voltage is limited to Step 2. Calculate the inductor ripple current: 40V. Therefore, if VFBP is higher than 1.235V, the output voltage will stay at 40V. (V )•200•10–6 I OUT + 1 – VIN RIPPLE = mA L 50 where VOUT is the desired output voltage. If the inductor 40 ripple current is less than the peak current, then the circuit will only operate in discontinuous conduction mode. The 30 TAGE (V) inductor value should be increased so that IRIPPLE < IPK. An application circuit can be designed to operate only in 20 discontinuous mode, but the output current capability OUTPUT VOL will be reduced. 10 Step 3. Calculate the average input current: 0 0 0.5 1 1.5 2 I FBP VOLTAGE (V) I RIPPLE mA 8410-1 F01 IN(AVG) = IPK – 2
Figure 1. FBP to VOUT Transfer Curve
84101fc For more information www.linear.com/LT8410 9 Document Outline Description Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Features Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts