cs51412g ON Semiconductor, cs51412g Datasheet - Page 11

• Manufacturer Part Number: cs51412g
• Description: 1.5a, 260 Khz And 520 Khz, Low Voltage Buck Regulators With External Bias Or Synchronization Capability
• Manufacturer: ON Semiconductor

where:
5.0 ms which is adequate to avoid any current stresses.
Figure 15 shows the gradual rise of the V
of the V
after the output voltage reaches the regulation. If the supply
voltage rises slower than the V
overshoot.
Short Circuit
Threshold, the regulator reduces the peak current limit by
40% and switching frequency to 1/4 of the nominal
frequency. These features are designed to protect the IC and
external components during overload or short circuit
conditions. In those conditions, peak switching current is
clamped to the current limit threshold. The reduced
switching frequency significantly increases the ripple
current, and thus lowers the DC current. The short circuit can
cause the minimum duty cycle to be limited by Minimum
Output Pulse Width. The foldback frequency reduces the
minimum duty cycle by extending the switching cycle. This
protects the IC from overheating, and also limits the power
that can be transferred to the output. The current limit
foldback effectively reduces the current stress on the
inductor and diode. When the output is shorted, the DC
current of the inductor and diode can approach the current
limit threshold. Therefore, reducing the current limit by 40%
can result in an equal percentage drop of the inductor and
The Soft-Start duration can be calculated by
V
C
I
Using a 0.1 mF C
When the V
SOURCE
Figure 15. The Power Up Transition of CS5141X
COMP
C
= V
equal to error amplifier's reference voltage.
SW
C
= Compensation capacitor connected to the V
pin steady-state voltage, which is approximately
= Output Source Current of the error amplifier.
during power up. There is no voltage overshoot
FB
T SS +
COMP
pin voltage drops below Foldback
Regulator
, the calculation shows a T
V C
I SOURCE
C
C COMP
pin, output voltage may
CS51411, CS51412, CS51413, CS51414
C
, V
O
and envelope
SS
http://onsemi.com
C
over
pin
11
diode current. The short circuit waveforms are captured in
Figure 16, and the benefit of the foldback frequency and
current limit is self-evident.
Thermal Considerations
necessary prior to the adoption of the regulator. The current
drawn by the IC includes quiescent current, predriver
current, and power switch base current. The quiescent
current drives the low power circuits in the IC, which
include comparators, error amplifier and other logic blocks.
Therefore, this current is independent of the switching
current and generates power equal to
where:
switch and is approximately equal to 12 mA in worst case.
During steady state operation, the IC draws this current from
the Boost pin when the power switch is on and then receives
it from the V
current always returns to the V
current goes out to the regulator's output even when the
power switch is turned off, a minimum load is required to
prevent overvoltage in light load conditions. If the Boost pin
voltage is equal to V
dissipation due to predriver current can be calculated by
current divided by beta of the device. Beta of 60 is used here
to estimate the base current. The Boost pin provides the base
current when the transistor needs to be on.
A calculation of the power dissipation of the IC is always
I
The predriver current is used to turn on/off the power
The base current of a bipolar transistor is equal to collector
Figure 16. In Short Circuit, the Foldback Current and
Foldback Frequency Limit the Switching Current to
Q
= quiescent current.
Protect the IC, Inductor and Catch Diode
W DRV + 12 mA
IN
pin when the switch is off. The predriver
IN
W Q + V IN
+ V
O
when the switch is on, the power
(V IN * V O )
SW
pin. Since the predriver
I Q
V O 2
V IN
)