LT1339C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1339C' PDF : 20 Pages View PDF
LT1339
APPLICATIONS INFORMATION
RSENSE Selection for Output Current
RSENSE generates a voltage that is proportional to the
inductor current for use by the LT1339 current sense
amplifier. The value of RSENSE is based on the required
load current. The average current limit function has a
typical threshold of 120mV/RSENSE, or:
RSENSE = 120mV/ILIMIT
Operation with VSENSE 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 VFB (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 VFB pin when the output is at its desired value.
The output voltage is thus set following the relation:
VOUT = 1.25(1 + R2/R1)
when an external resistor divider is connected to the
output as shown in Figure 1.
VOUT
R2
LT1339 VFB 9
SGND
R1
8
1339 • F01
Figure 1. Programming LT1339 Output Voltage
If high value feedback resistors are used, the input bias
current of the VFB pin (1µA maximum) could cause a slight
increase in output voltage. A Thevenin resistance at the
VFB pin of <5k is recommended.
Oscillator Components RCT and CCT
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
the minimum off-time of the PWM controller. This limits
maximum duty cycle (DCMAX) to:
DCMAX = 1 – (tDISCH)(fO)
This relation corresponds to the minimum value of the
timing resistor (RCT), which can be determined according
to the following relation (RCT vs DCMAX graph appears in
the Typical Performance Characteristics section):
RCT(MIN) ≈ [(0.8)(10 –3)(1 – DCMAX)] –1
Values for RCT > 15k yield maximum duty cycles above
90%. Given a timing resistor value, the value of the timing
capacitor (CCT) can then be determined for desired oper-
ating frequency (fO) using the relation:
( () ( ) )( ) ( ) CCT ≈
1/ fO
−
100
10 −9
RCT / 1.85
+
1.75
2.5
10−3
−
3.375 /RCT
A plot of Operating Frequency vs RCT and CCT is shown in
Figure 2. Typical 100kHz operational values are CCT =
1000pF and RCT = 16.9k.
160
140
CCT = 1.0nF
120
CCT = 1.5nF
100
80
60
40 CCT = 3.3nF
20
CCT = 2.2nF
0
0
5 10 15 20 25 30
TIMING RESISTOR (kΩ)
LT1339 • F02
Figure 2. Oscillator Frequency vs RCT, CCT
Average Current Limit
The average current limit function is implemented using
an external capacitor (CAVG) connected from IAVG to SGND
that forms a single pole integrator with the 50kΩ output
10
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