MAX5068
optimally compensated, the current loop responds to
input-voltage transients within one cycle.
Current Limit
The current-sense resistor (R
CS
), connected between
the source of the MOSFET and ground, sets the current
limit. The CS input has a voltage trip level (V
CS
) of
314mV. Use the following equation to calculate the
value of R
CS
:
where I
PRI
is the peak current in the primary that flows
through the MOSFET at full load.
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (NDRV) quickly
terminates the current on-cycle. In most cases, a small
RC filter is required to filter out the leading-edge spike
on the sense waveform. Set the corner frequency to a
few MHz above the switching frequency.
Applications Information
Layout Recommendations
Keep all PC board traces carrying switching currents
as short as possible, and minimize current loops.
For universal AC input design, follow all applicable safe-
ty regulations. Offline power supplies may require UL,
VDE, and other similar agency approvals. Contact these
agencies for the latest layout and component rules.
Typically, there are two sources of noise emission in a
switching power supply: high di/dt loops and high dv/dt
surfaces. For example, traces that carry the drain cur-
rent often form high di/dt loops. Similarly, the heatsink of
the MOSFET presents a dv/dt source, thus minimize the
surface area of the heatsink as much as possible.
To achieve best performance and to avoid ground
loops, use a solid ground-plane connection.
