06-07-2012, 03:05 PM
600kHz 3A Step-Down Switching Regulator
600khz 3AMP step down regulator.pdf (Size: 528.24 KB / Downloads: 44)
DESCRIPSION
The TJ4519 is a current mode switching regulator with an integrated switch, operating at 600kHz with enable
functions. The integrated switch allows for cost effective low power solutions (peak switch current 3 amps).
High frequency of operation allows for very small passive components. Current mode operation allows for fast
dynamic response and instantaneous duty cycle adjustment as the input varies (ideal for CPE applications
where the input is a wall plug power). The low shutdown current makes it ideal for portable applications where
battery life is important.
APPLICATION INFORMATION
The TJ4519 is a current mode buck converter regulator. TJ4519 has an internal fixed-frequency clock. The
TJ4519 uses two feedback loops that control the duty cycle of the internal power switch. The error amplifier
functions like that of the voltage mode converter. The output of the error amplifier works as a switch current
reference. This technique effectively removes one of the double poles in the voltage mode system. With this, it is
much simpler to compensate a current mode converter to have better performance. The current sense amplifier in
the TJ4519 monitors the switch current during each cycle. Overcurrent protection (OCP) is triggered when the
current limit exceeds the upper limit of 3A, detected by a voltage on COMP greater than about 2V. When an OCP
fault is detected, the switch is turned off and the external COMP capacitor is quickly discharged using an internal
npn transistor. Once the COMP voltage has fallen below 250mV, an internal timer prevents any operation for 50μs,
after which the part enters a normal startup cycle.
Enable
Pulling and holding the EN pin below 0.4V activates the shut down mode of the TJ4519 which reduces the input
supply current to less than 10uA. During the shut down mode, the switch is turned off. The TJ4519 is turned on if
the EN pin is pulled high.
Oscillator
Its internal free running oscillator sets the PWM frequency at 600 kHz for the TJ4519 without any external
components to program the frequency.
UVLO
When the EN pin is pulled and held above 1.8Vm, the voltage on Pin IN determines the operation of the TJ4519.
As VIN increases during power up, the internal circuit senses VIN and keeps the power transistor off until VIN
reaches 2.6V
Load current
The peak current IPEAK in the switch is internally limited. For a specific application, the allowed load current
ICMAX will change if the input voltage drifts away from the original design as given for continuous current mode:
Inductor Selection
The factors for selecting the inductor include its cost, efficiency, size and EMI. For a typical TJ4519 application,
the inductor selection is mainly based on its value, saturation current and DC resistance. Increasing the inductor
value will decrease the ripple level of the output voltage while the output transient response will be degraded. Low
value inductors offer small size and fast transient responses while they allow large ripple currents, poor
efficiencies and require more output capacitance for low output ripple. The inductor should be able to handle the
peak current without saturating and its copper resistance in the winding should be as low as possible to minimize
its resistive power loss. A good trade-off among its size, loss and cost is to set the inductor ripple current to be
within 15% to 30% of the maximum output current.