LT1432 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
LT1432
APPLICATI S I FOR ATIO
voltage. This extra sense voltage is set by output voltage
and R4 under normal loads, but drops to near zero when
the output is shorted.
The 40µA bias current flowing out of the VLIM pin must be
accounted for when calculating a value for R4. This current
flows through R3, causing a 4mV decrease in sense
voltage for R3 = 100Ω. The following formulas define
current limit conditions:
Current limit at VOUT = 5V
( ) ( ) ( ) 60mV – IB
=
R3
+
VOUT
R3
R4
–
RSENSE
IRIP
2
RSENSE
Short Circuit Current = 60mV – IB(R3)
RSENSE
R SENSE
=
VLIM
IMAX (1 .2)
R4 =
( ) VOUT R3
( ) ( ) VS – 60mV + IB
R3
+
RSENSE
IRIP
2
VS = Desired full load sense voltage.
IMAX = Peak load current (for any time greater than
50µs)
IB = VLIM pin bias current (≈40mA)
To maintain high efficiency and avoid any startup prob-
lems with loads that have non-linear V/I characteristics, a
100mV (average) sense voltage is suggested for foldback
current limiting. The suggested value for R3 is 100Ω. This
is a compromise value to keep errors due to VLIM bias
current low, and to minimize current drain on the output
created by the R3/R4 path. From the previous design
example, with IMAX = 2A and IRIP/2 = 0.55A, and assuming
R3 = 100Ω, VLIM = 100mV:
( ) RSENSE
=
100mV = 0.042Ω
2A 1.2
( ) 5V 100Ω
R4 =
( ) ( ) 100mV – 60mV + 100Ω 40µA – 0.042 0.55
= 7.45kΩ
Current limit at VOUT = 5V
( ) ( )( ) 60mV – 40µA 100Ω
=
+
5V
100
7.45k
0.042
0.55
0.042Ω
= 2.38A
Current limit (output shorted)
( ) 60mV – 100Ω 40µA
=
= 1.33A
0.042Ω
Minimum Input Voltage
Minimum input voltage for a buck converter using the
LT1432 is actually limited by the IC switcher used with it.
There are three factors which contribute to the minimum
voltage. At very light loads, the charge pump technique
used to provide the floating power for the switcher chip is
unable to provide sufficient current. See Figure 16 for the
minimum load required as a function of input voltage
when operating in the normal mode.
At moderate to heavy loads, switch on-resistance and
maximum duty cycle will limit minimum input voltage.
Graphs in the Typical Performance Characteristics section
show minimum input voltage as a function of load current.
At moderate loads, maximum switch duty cycle is the
limiting factor. The LT1070 family, operating at 40kHz has
a maximum duty cycle of about 94%. The LT1170 family
runs at 100kHz and has a maximum duty cycle of 90%. The
LT1270 and LT1271 operate at 60kHz with a maximum
duty cycle of 92%. The curves were generated using the
expected worst case duty cycle for these devices over the
commercial operating temperature range (0°C to 100°C
junction temperature). Note that the lower frequency
devices will operate at lower input voltage because of their
higher duty cycle. These devices will require larger induc-
tors, however. (Yet another example of the universal “no
free lunch” syndrome).
15
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