Datasheet

0.1 PF
50
LM5033
V
IN
Supply
Voltage
LM5033
www.ti.com SNVS181B –APRIL 2004–REVISED APRIL 2013
Figure 13. Input Transient Protection
If V
CC
is not powered externally, requiring all internal bias currents for the LM5033, and output driver currents, to
be supplied at Vin and through the internal regulator, the required input current (Iin) is shown in the Typical
Performance Characteristics (I
IN
vs. V
IN
).
If V
CC
is powered externally, Iin will increase with V
IN
as shown in the above mentioned graph until the external
voltage is applied to V
CC
. In most applications, this occurs once the outputs are enabled and load current begins
to flow. The current into Vin will then drop to a nominal 150µA (Pin 10 = open or grounded).
V
CC
(PIN 4)
The capacitor at the V
CC
pin provides not only noise filtering and stability, but also a necessary time delay during
start-up. The time delay allows the internal circuitry of the LM5033, and associated external circuitry, to stabilize
before V
CC
reaches its final value, at which time the outputs are enabled and the softstart sequence begins. Any
external circuitry connected to the REF output (Pin 2) and Softstart (Pin 10) should be designed to stabilize
during the time delay.
The current limit of the V
CC
regulator, and the external capacitor, determine the V
CC
turn-on time delay. Typically,
a 1µF capacitor will provide approximately 300 µs of delay, with larger capacitors providing proportionately longer
delays. Experimentation with the final design may be necessary to determine the minimum value for the V
CC
capacitor.
SOFTSTART (PIN 10)
The capacitor at pin 10 determines the time required for the output duty cycle to increase from zero to the final
value for regulation. The minimum acceptable time is dependent on the response of the feedback loops to the
COMP pin, as well as the characteristics of the magnetic components. If the Softstart time is too quick, the
system output could significantly overshoot its intended voltage before the loop has a chance to establish
regulation, possibly adversely affecting the load. Experimentation with the final design is usually necessary to
determine the minimum value for the SS capacitor.
CURRENT SENSE (PIN 8)
This pin typically receives an input representative of the primary current from the current sense elements of the
external circuitry. The peak amplitude at this pin must be less than 0.5V for normal operation. Filtering at this pin
should be sufficient to prevent false triggering of the Current Sense comparator, but not significantly delay
detection of an overcurrent condition. The filter’s capacitor at pin 8 should not be larger than 2200 pF.
OSCILLATOR, SYNC INPUT (PIN 9)
The internal oscillator frequency is generally selected in conjunction with the system magnetic components, and
any other aspects of the system which may be affected by the frequency. The R
t
resistor at pin 9 sets the
frequency according to the formula in the Functional Description. Each output (OUT1 and OUT2) switches at half
the oscillator frequency. If the required frequency value is critical in a particular application, the tolerance of the
external resistor, and the frequency tolerance indicated in the Electrical Characteristics, must be taken into
account when selecting the resistor.
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