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UC1848 Datasheet(PDF) 5 Page - Texas Instruments

Part No. UC1848
Description  Average Current Mode PWM Controller
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
Logo TI - Texas Instruments

UC1848 Datasheet(HTML) 5 Page - Texas Instruments

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A capacitor from the CT pin to GND programs oscillator
frequency, as shown in Fig. 2. Frequency is determined
F = 1 / (10k
• CT).
The sawtooth wave shape is generated by a charging
current of 200 A and a discharge current of 1800 A. 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 40k . This will allow up to a 22%
decrease in frequency.
Inductor Current Waveform Synthesizer
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 cur-
rent, 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 (Fig. 4) con-
sists of a unidirectional voltage follower which forces the
voltage on capacitor CI to follow the on time switch cur-
rent waveform. A programmable discharge current syn-
thesizes 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
UC3848 is to translate maximum average inductor cur-
rent to a 4V signal level. Given N and Ns (the turns ratio
of the power and current sense transformers), proper
scaling of IL to V(CI) requires a sense resistor Rs as cal-
culated 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 A.
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.
There are several methods to program IOFF. If accurate
average current control is required during short circuit op-
eration, IOFF must track output voltage. The method
shown in Fig. 4 derives a voltage proportional to VIN • D
(Duty Cycle). (In a buck converter, output voltage is pro-
portional to VIN
• D.) A resistively loaded diode connec-
tion to the bootstrap winding yields a square wave whose
amplitude is proportional to VIN and is duty cycle modu-
lated by the control circuit. Averaging this waveform with
a filter generates a primary side replica of secondary reg-
ulated VO. A single pole filter is shown, but in practice a
two or three pole filter provides better transient response.
Filtered voltage is converted by ROFF to a current to the
IOFF pin to control CI down slope.
Figure 3: Error amplifier gain and phase response
over frequency.

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