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UC1548 Fiches technique(PDF) 5 Page - Texas Instruments

No de pièce UC1548
Description  Primary Side PWM Controller
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Fabricant  TI1 [Texas Instruments]
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UC1548 Fiches technique(HTML) 5 Page - Texas Instruments

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Figure 3: Error Amplifier Gain and Phase Response over Frequency
A capacitor from the CT pin to GND programs oscillator
frequency, as shown in Figure 2. Frequency is determined
F = 1 / (10k
• CT).
The sawtooth wave shape is generated by a charging cur-
rent of 200
µA and a discharge current of 1800µA. The
discharge time of the sawtooth is guaranteed dead time
for the output driver. If the maximum duty cycle control is
defeated by connecting DMAX to VREF, the maximum
duty cycle is limited by the oscillator to 90%. If an adjust-
ment is required, an additional trim resistor RT from CT to
ground can be used to adjust the oscillator frequency. RT
should not be less than 40kohms. This will allow up to a
22% decrease in frequency.
Average current mode control is a very useful technique
to control the value of any current within a switching con-
verter. Input current, output inductor current, switch
current, diode current or almost any other current can be
controlled. In order to implement average current mode
control, the value of the current must be explicitly known
at all times. To control output inductor current (IL) in a
buck derived isolated converter, switch current provides
inductor current information, but only during the on time of
the switch. During the off time, switch current drops
abruptly to zero, but the inductor current actually dimin-
ishes with a slope dIL/dt =
−Vo/L. This down slope must
be synthesized in some manner on the primary side to
provide the entire inductor current waveform for the con-
trol circuit.
The patented current waveform synthesizer (Figure 4)
consists of a unidirectional voltage follower which forces
the voltage on capacitor CI to follow the on time switch
current waveform. A programmable discharge current
synthesizes the off time portion of the waveform. ION is
the input to the follower. The discharge current is pro-
grammed at IOFF.
The follower has a one volt offset, so that zero current
corresponds to one volt at CI. The best utilization of the
UC3548 is to translate maximum average inductor current
to a 4 volt signal level. Given N and Ns (the turns ratio of
the power and current sense transformers respectively),
proper scaling of IL to V(CI) requires a sense resistor Rs
as calculated from:
Rs = 4V
• Ns • N / IL(max).
Restated, the maximum average inductor current will be
limited to:
IL(max) = 4V
• Ns • N/Rs.
IOFF and CI need to be chosen so that the ratio of
dV(CI)/dt to dIL/dt is the same during switch off time as
on time. Recommended nominal off current is 100
This requires
CI = (100
µA • N • Ns • L) / (Rs • Vo(nom))
where L is the output inductor value and Vo(nom) is the
converter regulated output voltage.

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