Datasheet

10
LT1339
sn1339 1339fas
APPLICATIONS INFORMATION
WUU
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R
SENSE
Selection for Output Current
R
SENSE
generates a voltage that is proportional to the
inductor current for use by the LT1339 current sense
amplifier. The value of R
SENSE
is based on the required
load current. The average current limit function has a
typical threshold of 120mV/R
SENSE
, or:
R
SENSE
= 120mV/I
LIMIT
Operation with V
SENSE
common mode voltage below 4.5V
may slightly degrade current limit accuracy. See Average
Current Limit Threshold Tolerance vs Common Mode
Voltage curve in the Typical Performance Characteristics
section for more information.
Output Voltage Programming
Output voltage is programmed through a resistor feed-
back network to V
FB
(Pin 9) on the LT1339. This pin is the
inverting input of the error amplifier, which is internally
referenced to 1.25V. The divider is ratioed to provide
1.25V at the V
FB
pin when the output is at its desired value.
The output voltage is thus set following the relation:
V
OUT
= 1.25(1 + R2/R1)
when an external resistor divider is connected to the
output as shown in Figure 1.
the minimum off-time of the PWM controller. This limits
maximum duty cycle (DC
MAX
) to:
DC
MAX
= 1 – (t
DISCH
)(f
O
)
This relation corresponds to the minimum value of the
timing resistor (R
CT
), which can be determined according
to the following relation (R
CT
vs DC
MAX
graph appears in
the Typical Performance Characteristics section):
R
CT(MIN)
≈ [(0.8)(10
–3
)(1 – DC
MAX
)]
–1
Values for R
CT
> 15k yield maximum duty cycles above
90%. Given a timing resistor value, the value of the timing
capacitor (C
CT
) can then be determined for desired oper-
ating frequency (f
O
) using the relation:
C
f
R
R
CT
O
CT
CT
≈
()
−
()




()
+
()




−
()
−
−
1 100 10
185
175
2 5 10 3 375
9
3
/
/.
.
../
A plot of Operating Frequency vs R
CT
and C
CT
is shown in
Figure 2. Typical 100kHz operational values are C
CT
=
1000pF and R
CT
= 16.9k.
R1
R2
V
OUT
1339 • F01
V
FB
SGND
LT1339
9
8
Figure 1. Programming LT1339 Output Voltage
If high value feedback resistors are used, the input bias
current of the V
FB
pin (1µA maximum) could cause a slight
increase in output voltage. A Thevenin resistance at the
V
FB
pin of <5k is recommended.
Oscillator Components R
CT
and C
CT
The LT1339 oscillator creates a modified sawtooth wave
at its timing node (CT) with a slow charge, rapid discharge
characteristic. The rapid discharge time corresponds to
Figure 2. Oscillator Frequency vs R
CT
, C
CT
Average Current Limit
The average current limit function is implemented using
an external capacitor (C
AVG
) connected from I
AVG
to SGND
that forms a single pole integrator with the 50kΩ output
TIMING RESISTOR (kΩ)
0
OSCILLATOR FREQUENCY (kHz)
15
25
LT1339 • F02
510 20
160
140
120
100
80
60
40
20
0
30
C
CT
= 3.3nF
C
CT
= 2.2nF
C
CT
= 1.5nF
C
CT
= 1.0nF