SM74203 Datasheet, PDF (11/24 Pages) Texas Instruments – SM74203 60V Low Side Controller for Boost and SEPIC

resistor to create a minimum compensation ramp with a slope
of 100 mV x f_SW (typical). The slope of the compensation ramp
increases when external resistance is added for filtering the
current sense (R_S1) or in the position R_S2. As shown in Figure
4 and the block diagram, the sensed current slope and the
compensation slope add together to create the signal used
for current limiting and for the control loop itself.
FIGURE 4. Slope Compensation
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In peak current mode control the optimal slope compensation
is proportional to the slope of the inductor current during the
power switch off-time. For boost converters the inductor cur-
rent slope while the MOSFET is off is (V_O - V_IN) / L. This
relationship is combined with the requirements to set the peak
current limit and is used to select R_SNS and R_S2 in the Design
Considerations section.
SOFT-START
The soft-start feature allows the power converter output to
gradually reach the initial steady state output voltage, thereby
reducing start-up stresses and current surges. At power on,
after the VCC and input under-voltage lockout thresholds are
satisfied, an internal 10 µA current source charges an external
capacitor connected to the SS pin. The capacitor voltage will
ramp up slowly and will limit the COMP pin voltage and the
switch current.
MOSFET GATE DRIVER
The SM74203 provides an internal gate driver through the
OUT pin that can source and sink a peak current of 1A to
control external, ground-referenced N-channel MOSFETs.
THERMAL SHUTDOWN
Internal thermal shutdown circuitry is provided to protect the
SM74203 in the event that the maximum junction temperature
is exceeded. When activated, typically at 165°C, the controller
is forced into a low power standby state, disabling the output
driver and the VCC regulator. After the temperature is re-
duced (typical hysteresis is 25°C) the VCC regulator will be
re-enabled and the SM74203 will perform a soft-start.
Design Considerations
The most common circuit controlled by the SM74203 is a non-
isolated boost regulator. The boost regulator steps up the
input voltage and has a duty ratio D of:
The following is a design procedure for selecting all the com-
ponents for the boost converter circuit shown in Figure 1. The
application is "in-cabin" automotive, meaning that the oper-
ating ambient temperature ranges from -20°C to 85°C. This
circuit operates in continuous conduction mode (CCM),
where inductor current stays above 0A at all times, and de-
livers an output voltage of 40.0V ±2% at a maximum output
current of 0.5A. Additionally, the regulator must be able to
handle a load transient of up to 0.5A while keeping V_O within
±4%. The voltage input comes from the battery/alternator
system of an automobile, where the standard range 9V to 16V
and transients of up to 32V must not cause any malfunction.
SWITCHING FREQUENCY
The selection of switching frequency is based on the tradeoffs
between size, cost, and efficiency. In general, a lower fre-
quency means larger, more expensive inductors and capac-
itors will be needed. A higher switching frequency generally
results in a smaller but less efficient solution, as the power
MOSFET gate capacitances must be charged and dis-
charged more often in a given amount of time. For this appli-
cation, a frequency of 500 kHz was selected as a good
compromise between the size of the inductor and efficiency.
PCB area and component height are restricted in this appli-
cation. Following the equation given for R_T in the Applications
Information section, a 33.2 kΩ 1% resistor should be used to
switch at 500 kHz.
MOSFET
Selection of the power MOSFET is governed by tradeoffs be-
tween cost, size, and efficiency. Breaking down the losses in
the MOSFET is one way to determine relative efficiencies be-
tween different devices. For this example, the SO-8 package
provides a balance of a small footprint with good efficiency.
Losses in the MOSFET can be broken down into conduction
loss, gate charging loss, and switching loss.
Conduction, or I²R loss, P_C, is approximately:
(V_D is the forward voltage drop of the output diode)
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