TC120_13 Datasheet, PDF (6/16 Pages) Microchip Technology – PWM/PFM Step-Down Combination Regulator/Controller

TC120
3.6 Low Power Shutdown Mode/Soft
Start Input
The SHDN/SS input acts as both the shutdown control
and the node for the external soft start capacitor, which
is charged by an internal 1A current source. A value
of 4700pF (100pF minimum) is recommended for the
soft start capacitor. Failure to do this may cause large
overshoot voltages and/or large inrush currents result-
ing in possible instability. The TC120 enters a low
power shutdown mode when SHDN/SS is brought low.
While in shutdown, the oscillator is disabled and the
output discharge switch is turned on, discharging the
output capacitor. Figure 3-3 shows the recommended
interface circuits to the SHDN/SS input. As shown, the
SHDN/SS input should be controlled using an open
collector (or open drain) device, such that the SHDN/
SS input is grounded for shutdown mode, and open-
circuited for normal operation (Figure 3-3a). If a CMOS
device is used to control shutdown (Figure 3-3b), the
value of R1 and C_SS should be chosen such that the
voltage on SHDN/SS rises from ground to 0.65V in
1.5msec (Figure 3-4). If shutdown is not used, C_SS
must still be connected as shown in Figure 3-3c and
Figure 3-3d. SHDN/SS may be pulled up with a resistor
(Figure 3-3c) as long as the values of R_SS and C_SS
provide the approximate charging characteristic on
power up shown in Figure 3-4. C_SS only may also be
connected as shown in Figure 3-3d with C_SS chosen at
4700pF (minimum 100pF).
3.7 Undervoltage Lockout (UVLO)
The TC120 is disabled whenever V_IN is below the
undervoltage lockout threshold. This threshold is equal
to the guaranteed minimum operating voltage for the
TC120 (i.e., 2.2V). When UVLO is active, the TC120 is
completely disabled.
3.8 Short Circuit Protection
Upon detection of an output short circuit condition, the
TC120 reduces the PWM duty cycle to a minimum
value using its internal protection timer. The sequence
of events is as follows: when an output voltage
decrease to near zero is detected (as the result of an
overload), the internal (5msec) protection timer is
started. If the output voltage has not recovered to
nominal value prior to the expiration of the protection
timer, the TC120 is momentarily shut down by
dedicated, internal circuitry. Immediately following this
action, the soft start sequence is engaged in an attempt
to re-start the TC120. If the output short circuit is
removed, normal operation is automatically restored. If
the short circuit is still present, the timed self-shutdown
sequence described above is repeated.
3.9 External Switching Transistor
Selection
EXT is a complimentary output with a maximum ON
resistances of 32 to V_DD when high and 26 to
ground when low, at V_OUT = 5V. It is designed to
directly drive a P-channel MOSFET (Figure 3-5). The
P-channel MOSFET selection is determined mainly by
the on-resistance, gate-source threshold and gate
charge requirements. Also, the drain-to-source and
gate-to-source breakdown voltage ratings must be
greater than V_INMAX. The total gate charge specification
should be less than 100nC for best efficiency. The
MOSFET must be capable of handling the required
peak inductor current, and should have a very low on-
resistance at that current. For example, a Si9430
MOSFET has a drain-to-source rating of -20V, and a
typical on-resistance r_DSON of 0.07 at 2A, with V_GS =
-4.5V. (EXTW (Figure 3-6) may be gated with external
circuitry to add blanking, or as an auxiliary timing
signal.) Table 3-1 lists suggested components and
suppliers.
3.10 Board Layout Guidelines
As with all inductive switching regulators, the TC120
generates fast switching waveforms, which radiate
noise. Interconnecting lead lengths should be
minimized to keep stray capacitance, trace resistance
and radiated noise as low as possible. In addition, the
GND pin, input bypass capacitor and output filter
capacitor ground leads should be connected to a single
point. The input capacitor should be placed as close to
power and ground pins of the TC120 as possible. The
length of the EXT trace must also be kept as short as
possible.
DS21365C-page 6
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