MAX5052/MAX5053
Current-Mode PWM Controllers with an Error
Amplifier for Isolated/Nonisolated Power Supplies
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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
291mV. 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.
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, typically within 60ns.
In most cases, a small RC filter is required to filter out
the leading-edge spike on the sense waveform. Set the
corner frequency at a few megahertz.
Applications Information
Primary Regulated, Isolated
Telecom Power Supply
Figure 5 shows a complete design of a dual-output power
supply with a telecom voltage range of 36V to 72V. An
important aspect of this power supply is its primary-side
regulation. This regulation, through the tertiary winding,
also acts as bias winding for the MAX5052.
In the circuit of Figure 5, cross-regulation has been
improved (tertiary and 5V outputs) by using chip induc-
tors, L1 and L2, and R7||R2. R7||R2 presents enough
loading on the tertiary winding output to allow ±5% load
regulation on the 5V output over a load current range
from 150mA to 1.5A.
Layout Recommendations
All printed circuit board traces carrying switching cur-
rents must be kept as short as possible, and the cur-
rent loops they form must be minimized. The pins of the
µMAX package have been placed to allow easy inter-
facing to the external MOSFET.
For universal AC input design, all applicable safety reg-
ulations must be followed. Offline power supplies may
require UL, VDE, and other similar agency approvals.
These agencies can be contacted 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 the sur-
face area of the heatsink must be minimized as much
as possible.
To achieve best performance, a star ground connection
is recommended to avoid ground loops. For example,
the ground returns for the power-line input filter, power
MOSFET switch, and sense resistor should be routed
separately through wide copper traces to meet at a sin-
gle-system ground connection.
Chip Information
TRANSISTOR COUNT: 1449
PROCESS: BiCMOS
