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SC202A
Applications Information
General Description
The SC202A is a synchronous step-down PWM (Pulse
Width Modulated) DC-DC regulator utilizing a 3.5MHz
fixed-frequency voltage-mode architecture and an inter-
nal 1μH inductor. The device is designed to operate in
fixed-frequency PWM mode and enter PSAVE (power save)
mode utilizing pulse frequency modulation under light
load conditions for maximizing efficiency. Two capacitors
are the only external components required — one for
input decoupling and one for output filtering. The output
voltage is programmable, eliminating the need for exter-
nal programming resistors. Loop compensation is also
internal, eliminating the need for external components to
control stability.
Programmable Output Voltage
The SC202A has 15 fixed output voltage levels which can
be individually selected by programming the CTL control
pins (CTL3-0 — see Table 1 on page 2 for settings). The
device is disabled whenever all four CTL pins are pulled
low and enabled whenever at least one of the CTL pins is
pulled high. This configuration eliminates the need for a
dedicated enable pin. Each CTL pin is internally pulled
down via 1MΩ if V IN is below 1.5V or if the voltage on the
control pin is below the input high voltage. This ensures
that the output is disabled when power is applied if there
are no inputs to the CTL pins. Each weak pull-down is dis-
abled whenever its pin is pulled high and remains disabled
until all CTL pins are pulled low.
The output voltage can be set using different methods. If
a static output voltage is required, the CTL pins can be
tied to either IN or GND to set the desired voltage when-
ever power is applied at IN. If enable control is required,
each CTL pin can be tied to either GND or to a micropro-
cessor I/O line to create the desired control code whenever
the control signal is forced high. This method is equivalent
to using the CTL pins collectively as a single enable pin. A
third option is to connect each of the four CTL pins to
individual microprocessor I/O lines. Any of the 15 output
voltages can be programmed using this method. If only
two output voltages are needed, the CTL pins can be com-
bined in a way that will reduce the number of I/O lines to
1, 2, or 3, depending on the control code for each desired
voltage.
Dynamic Output Voltage Adjustment
Dynamically changing the CTL pins allows dynamic
voltage adjustment for systems that reduce the supply
voltage when entering sleep states. This should done
using specific procedures. Attention needs to be made so
that applying all zeros in a very short period to the CTL
pins when changing the output voltage will temporarily
disable the device. Therefore it is important to avoid
these combinations of all zero when dynamically chang-
ing the CTL levels. For example, when the CTLs change
from 0001 to 0010 (0.8V to 1.2V), a transitional state of
0000 (shut down) state might occur in a very short period
of time, which could result the device to be disabled
unintentionally. In order to achieve such operation, the
correct logic transition stages should be 001000110001
(1V1.2V0.8V). The 0011 (1.2V) state should be kept short
to prevent from the 0000 state.
If the output voltage level is not within the specified the
CTL voltage levels, the resistor divider ratio can be
switched by a logic voltage level through an external
MOSFET to achieve the dynamic output voltage
transition.
Secondly, when the CTL pin is toggled for the output
voltage to increase, the regulator will increase the induc-
tor current and force the output voltage follow. When
the CTL pin is toggled for the output voltage to decrease,
the regulator will force the output voltage go down
immediately (at heavy load conditions) when the device
is in CCM. If the device is in DCM operation (the inductor
current does not go negative) then the output voltage
will go down as the load current drains the output
capacitor.
Adjustable Output Voltage Selection
If an output voltage other than one of the 15 program-
mable settings is needed, an external resistor divider
network can be added to the SC202A to adjust the output
voltage setting. This network scales the output based on
the resistor ratio and the programmed output setting.
12
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