SM74203 Datasheet, PDF (9/24 Pages) Texas Instruments – SM74203 60V Low Side Controller for Boost and SEPIC

Example Circuit
FIGURE 1. Design Example Schematic
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Applications Information
OVERVIEW
The SM74203 is a low-side N-channel MOSFET controller
that contains all of the features needed to implement single
ended power converter topologies. The SM74203 includes a
high-voltage startup regulator that operates over a wide input
range of 6V to 60V. The PWM controller is designed for high
speed capability including an oscillator frequency range up to
2 MHz and total propagation delays less than 100 ns. Addi-
tional features include an error amplifier, precision reference,
input under-voltage lockout, cycle-by-cycle current limit, slope
compensation, soft-start, oscillator sync capability and ther-
mal shutdown.
The SM74203 is designed for current-mode control power
converters that require a single drive output, such as boost
and SEPIC topologies. The SM74203 provides all of the ad-
vantages of current-mode control including input voltage
feed-forward, cycle-by-cycle current limiting and simplified
loop compensation.
HIGH VOLTAGE START-UP REGULATOR
The SM74203 contains an internal high-voltage startup reg-
ulator that allows the VIN pin to be connected directly to line
voltages as high as 60V. The regulator output is internally
current limited to 35 mA (typical). When power is applied, the
regulator is enabled and sources current into an external ca-
pacitor, C_F, connected to the VCC pin. The recommended
capacitance range for C_F is 0.1 µF to 100 µF. When the volt-
age on the VCC pin reaches the rising threshold of 5V, the
controller output is enabled. The controller will remain en-
abled until VCC falls below 4.7V. In applications using a
transformer, an auxiliary winding can be connected through
a diode to the VCC pin. This winding should raise the VCC
pin voltage to above 7.5V to shut off the internal startup reg-
ulator. Powering VCC from an auxiliary winding improves
conversion efficiency while reducing the power dissipated in
the controller. The capacitance of C_F must be high enough
that it maintains the VCC voltage greater than the VCC UVLO
falling threshold (4.7V) during the initial start-up. During a fault
condition when the converter auxiliary winding is inactive, ex-
ternal current draw on the VCC line should be limited such
that the power dissipated in the start-up regulator does not
exceed the maximum power dissipation capability of the con-
troller.
An external start-up or other bias rail can be used instead of
the internal start-up regulator by connecting the VCC and the
VIN pins together and feeding the external bias voltage (7.5V
to 14V) to the two pins.
INPUT UNDER-VOLTAGE DETECTOR
The SM74203 contains an input Under Voltage Lock Out
(UVLO) circuit. UVLO is programmed by connecting the UV-
LO pin to the center point of an external voltage divider from
VIN to GND. The resistor divider must be designed such that
the voltage at the UVLO pin is greater than 1.25V when VIN
is in the desired operating range. If the under voltage thresh-
old is not met, all functions of the controller are disabled and
the controller remains in a low power standby state. UVLO
hysteresis is accomplished with an internal 20 µA current
source that is switched on or off into the impedance of the set-
point divider. When the UVLO threshold is exceeded, the
current source is activated to instantly raise the voltage at the
UVLO pin. When the UVLO pin voltage falls below the 1.25V
threshold the current source is turned off, causing the voltage
at the UVLO pin to fall. The UVLO pin can also be used to
implement a remote enable / disable function. If an external
transistor pulls the UVLO pin below the 1.25V threshold, the
converter will be disabled. This external shutdown method is
shown in Figure 2.
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