600kHz 3A Step-Down Switching Regulator
APPLICATION INFORMATION
TJ4519
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. In the case of sustained overcurrent or dead-short, the part will
continually cycle through the retry sequence as described above, at a rate dependent on the value of Ccomp.
During start up, the voltage on COMP rises roughly at the rate of dv/dt=120μA/Ccomp. Ccomp is the total
capacitance value attached to COMP. Therefore, the retry time for a sustained overcurrent can be approximately
calculated as:
T
retry
= C
comp
•
2V
+ 50us
120uA
Figure 1 shows the voltage on COMP during a sustained overcurrent condition.
2V
250mV
Figure 1. Voltage on COMP for Startup and OCP
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 I
PEAK
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:
Jul. 2010 – Preliminary
-5-
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
