Datasheet LT3480 (Analog Devices) - 9

LT3480
OPERATION
The LT3480 contains a power good comparator which trips The LT3480 has an overvoltage protection feature which when the FB pin is at 86% of its regulated value. The PG disables switching action when the VIN goes above 38V output is an open-collector transistor that is off when the typical (36V minimum). When switching is disabled, the output is in regulation, allowing an external resistor to pull LT3480 can safely sustain input voltages up to 60V. the PG pin high. Power good is valid when the LT3480 is enabled and VIN is above 3.6V.
APPLICATIONS INFORMATION FB Resistor Network Operating Frequency Tradeoffs
The output voltage is programmed with a resistor divider Selection of the operating frequency is a tradeoff between between the output and the FB pin. Choose the 1% resis- efficiency, component size, minimum dropout voltage, and tors according to: maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may  V R1=R2 OUT −1 be used. The disadvantages are lower efficiency, lower  0.79V  maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (f Reference designators refer to the Block Diagram. SW(MAX)) for a given application can be calculated as follows:
Setting the Switching Frequency V f D
+
VOUT SW(MAX)
= The LT3480 uses a constant frequency PWM architecture
t
( )
ON(MIN) VD
+
VIN – VSW
that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground. A table where VIN is the typical input voltage, VOUT is the output showing the necessary R voltage, V T value for a desired switching D is the catch diode drop (~0.5V) and VSW is the frequency is in Figure 1. internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to
SWITCHING FREQUENCY (MHz) RT VALUE (k
Ω
)
safely accommodate high VIN/VOUT ratio. Also, as shown 0.2 187 in the next section, lower frequency allows a lower dropout 0.3 121 0.4 88.7 voltage. The reason input voltage range depends on the 0.5 68.1 switching frequency is because the LT3480 switch has finite 0.6 56.2 0.7 46.4 minimum on and off times. The switch can turn on for a 0.8 40.2 minimum of ~150ns and turn off for a minimum of ~150ns. 0.9 34 Typical minimum on time at 25°C is 80ns. This means that 1.0 29.4 1.2 23.7 the minimum and maximum duty cycles are: 1.4 19.1 1.6 16.2
DCMIN
=
fSWtON(MIN)
1.8 13.3 2.0 11.5
DCMAX
=
1– fSWtOFF(MIN)
2.2 9.76 2.4 8.66 where fSW is the switching frequency, the tON(MIN) is the
Figure 1. Switching Frequency vs. R
minimum switch on time (~150ns), and the t
T Value
OFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased. 3480fe For more information www.linear.com/LT3480 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts