LT1533C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1533C' PDF : 20 Pages View PDF
LT1533
APPLICATIONS INFORMATION
RVC
2k
CVC
0.01µF
VC PIN
CVC2
4.7nF
1533 F03
turns ratio of the transformer. The turns ratio must be
large enough to ensure that the transformer can put out a
voltage equal to the output voltage plus the diode under
minimum input conditions.
Figure 3
To prevent irregular switching, VC pin ripple should be
kept below 50mVP-P. Worst-case VC pin ripple occurs at
maximum output load current and will also be increased if
poor quality (high ESR) output capacitors are used. The
addition of a 0.0047µF capacitor on the VC pin reduces
switching frequency ripple to only a few millivolts. A low
value for RVC will also reduce VC pin ripple, but loop phase
margin may be inadequate.
Magnetics
Design of magnetics is dependent on topology. The fol-
lowing details the design of the magnetics for a push-pull
converter. In this converter the transformer usually stores
little energy. The following equations should be consid-
ered as the starting point to building a prototype.
VIN
Q1A
Q1B
T1
1:N
DS1 VSEC
LO
DS2
+
VOUT
CO
1533 F04
Figure 4
The following definitions will be used:
VIN = Input supply voltage
VSW = Switch-on voltage
VOUT = Desired output voltage
IOUT = Output current
f = Oscillator frequency
VF = Forward drop of the rectifier
Duty cycle is the major defining equation for this topology.
Note that the output L and C basically filter the chopped
voltage so duty cycle controls output voltage. N is the
( ) N =
VOUT + VF
2 •DCMAX VIN(MIN) − VSW
DCMAX is the maximum duty cycle of each driver with
respect to the entire cycle which consists of two periods
(Q1A on and Q1B on). So the effective duty cycle is
2 • DCMAX. The controller, in general, determines maxi-
mum duty cycle. A 44% maximum duty cycle is a guaran-
teed value for this part.
Some Common Turns Ratios
VIN
VOUT
N
5 ±10%
12
3.6
5 ±10%
15
4.4
5 ±10%
3.3
1.1
Remember to add sufficient margin in the turns ratio to
account for IR drops in the transformer windings, worst-
case diode forward drop (VF) and switch-on voltage (VSW).
There are a number of ways to choose the inductance
value for LO. We suggest as a starting point that LO be
selected such that the converter is continuous at
IOUT(MAX)/4. If your minimum IOUT is higher than this, or
you are operating at low currents such that the IC and
components can handle higher peak currents, then use a
higher number.
Continuous operation occurs when the current in the
inductor never goes to zero. Discontinuous operation
occurs when the inductor current drops to zero before the
start of the next cycle and can occur with small inductors
and light loads. There is nothing inherently bad about
discontinuous operation, however, the converter control
and operation is somewhat different. The inductor is
smaller for discontinuous operation but the peak currents
in the switch, the transformer, the diodes, inductor and
capacitor will be higher. But for low power situations these
may not present a big constraint.
11
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