CS5124XD8 ON Semiconductor, CS5124XD8 Datasheet - Page 9

• Manufacturer Part Number: CS5124XD8
• Description: IC CTRLR PWM CURRENT MODE 8-SOIC
• Manufacturer: ON Semiconductor

Specifications of CS5124XD8

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 440kHz
• Duty Cycle: 85%
• Voltage - Supply: 7.6 V ~ 20 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 105°C
• Package / Case: 8-SOIC (3.9mm Width)
• Frequency-max: 440kHz
• Output Voltage: - 0.3 V to + 20 V
• Output Current: 200 mA
• Mounting Style: SMD/SMT
• Switching Frequency: 440 KHz
• Operating Supply Voltage: 7.7 V to 20 V
• Maximum Operating Temperature: + 135 C
• Fall Time: 25 ns
• Minimum Operating Temperature: - 40 C
• Rise Time: 45 ns
• Synchronous Pin: No
• Topology: Flyback, Forward
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

converter. If the second current sense threshold is tripped,
the converter will shut off and restart in Soft−Start mode
until the high current condition is removed. The dead time
after a second threshold overcurrent condition will primarily
be determined by the time required to charge the Soft−Start
cap from 0.275 V nominal to 1.32 V.
dv/dt is present at the current sense pin. The signal must be
fast enough to reach the second threshold before the first
threshold turns off the driver. This will normally happen if
the forward inductor saturates or when there is a shorted
load.
current sense resistor, or even an inductor that does not
saturate during heavy output currents can prevent the second
threshold from being reached. In this case the first current
sense threshold will trip during each cycle of high output
current conditions. The first threshold will limit output
current but some components, especially the output rectifier,
can overheat due to higher than normal average output
current.
Slope Compensation
excess of 50% require an artificial ramp to be added to the
current waveform or subtracted from the feedback
waveform. For the current loop to be stable the artificial
ramp must be equivalent to at least 50% of the inductor
current down slope and is typically chosen between 75% to
100% of the inductor down current down slope.
compensation ramp will be equal to a certain fraction of the
inductor down current slope use the Formula (4).
Internal Ramp
R SENSE
ramp equivalent to 100% of the current inductor down slope
at CS5124 nominal conditions, a 5.0 V output, a 200 mW
current sense resistor and a 4:1 transformer ratio yields
The second threshold will only be reached when a high
Excessive filtering of the current sense signal, a low value
Current mode converters operating at duty cycles in
To choose an inductor value such that the internal slope
Calculating the nominal inductor value for an artificial
1
Slope Value Factor + Inductor Value(H)
(V OUT ) V RECTIFIER )
N SECONDARY
N PRIMARY
http://onsemi.com
(4)
9
20 mV ms
than 50% of the inductor down under all conditions,
substitute the minimum internal slope compensation value
and use 0.5 for the slope compensation value. Then check
that the actual inductor value will always be greater than the
inductor value calculated.
Powering the CS5124 from a Transformer Winding
transformer winding to enable the converter to be operated
at high efficiency over a wide input range.
converter that uses a second flyback winding to power V
R4 improves V
the turn off spike. Once the turn off spike has subsided the
voltage of this winding is voltage proportional to the voltage
on the main flyback winding. This voltage is regulated
because the main winding is clamped by the regulated output
voltage.
used to power V
product of a forward converter would be constant over the
range of line voltages and load currents; and the transformer
inductance could be chosen to store the required level of
energy during each cycle to power V
flyback energy is not directly regulated it would remain
constant. Unfortunately in a real converter there are many
nonideal effects that degrade regulation. Transformer
inductance varies, converter frequency varies, energy stored
in primary leakage inductance varies with output current,
stray transformer capacitances and various parasitics all
effect the level of energy available for V
energy is provided to V
provide power and efficiency will be reduced. If too much
energy is provided V
If this approach is taken the circuit must be carefully
designed and component values must be controlled for good
regulation.
To check that the slope compensation ramp will be greater
There are numerous ways to power the CS5124 from a
The CS5124 application circuit in Figure 1 is a flyback
A flyback winding from a forward transformer can also be
1
(5.0 V ) 0.3 V)
CC
CC
regulation with load changes by snubbing
CC
. Ideally the transformer volt−second
rises and may damage the controller.
CC
, the bootstrapping circuit must
1
4
0.2 W
CC
1.0 + 13.2 mH
. Even though the
CC
. If too little
CC
(5)
.