Application Information: continued
threshold is independent of switching duty cycle. The max-
imum load current, given by the following formula under
continuous conduction mode, is less than the CURRENT
LIMIT due to the ripple current.
VO (VIN – VO)
2(L) (VIN) (fS)
IO(MAX) = ILIM
–
where fS = switching frequency
ILIM = current limit threshold
VO = output voltage
VIN = input voltage
L = inductor value
When the regulator runs under current limit, the subhar-
monic oscillation may cause low frequency oscillation, as
shown in Figure 4. Similar to current mode control, this
oscillation occurs at the duty cycle greater than 50% and
can be alleviated by using a larger inductor value. The cur-
rent limit threshold is reduced to FOLDBACK CURRENT
when the FB pin falls below FOLDBACK THRESHOLD.
This feature protects the IC and external components
under the power up or over-load conditions.
Figure 2: A CS51411 buck regulator is synced by an external 350kHz
pulse signal.
Power Switch and Current Limit
The collector of the built-in NPN power switch is connect-
ed to the VIN pin, and the emitter to the VSW pin. When the
switch turns on, the VSW voltage is equal to the VIN minus
switch SATURATION VOLTAGE. In the buck regulator,
the VSW voltage swings to one diode drop below ground
when the power switch turns off, and the inductor current
is commutated to the catch diode. Due to the presence of
high pulsed current, the traces connecting the VSW pin,
inductor and diode should be kept as short as possible to
minimize the noise and radiation. For the same reason, the
input capacitor should be placed close to the VIN pin and
the anode of the diode.
The saturation voltage of the power switch is dependent on
the switching current, as shown in Figure 3.
0.7
0.6
0.5
Figure 4: The regulator in current limit.
0.4
0.3
0.2
0.1
0
BOOST Pin
The BOOST pin provides base driving current for the
power switch. A voltage higher than VIN provides required
headroom to turn on the power switch. This in turn
reduces IC power dissipation and improves overall system
efficiency. The BOOST pin can be connected to an external
boost-strapping circuit which typically uses a 0.1µF capaci-
tor and a 1N914 or 1N4148 diode, as shown in Application
Diagram. When the power switch is turned on, the voltage
on the BOOST pin is equal to
0
0.5
1
1.5
Switching Current (A)
Figure 3: The saturation voltage of the power switch increases with the
conducting current.
V
BOOST = VIN + VO – VF
where VF = diode forward voltage.
Members of the CS5141X family contain pulse-by-pulse
current limiting to protect the power switch and external
components. When the peak of the switching current
reaches the CURRENT LIMIT, the power switch turns off
after the CURRENT LIMIT DELAY. The switch will not
turn on until the next switching cycle. The current limit
The anode of the diode can be connected to any DC voltage
other than the regulated output voltage. However, the
maximum voltage on the BOOST pin shall not exceed 40V.
6
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