AP5100_1011 Datasheet, PDF (8/10 Pages) Diodes Incorporated – 1.2A Step-Down Converter with 1.4MHz Switching Frequency

AP5100
1.2A Step-Down Converter with 1.4MHz Switching
Frequency
Applications Information (Continued)
Internal Soft Start
Soft start is traditionally implemented to prevent the
excess inrush current. This in turn prevents the converter
output voltage from overshooting when it reaches
regulation. The AP5100 has an internal current source
with a soft start capacitor to ramp the reference voltage
from 0V to 0.810V. The soft start time is internally fixed at
200us (TYP). The soft start sequence is reset when there
is a Thermal Shutdown, Under Voltage Lockout (UVLO) or
when the part is disabled using the EN pin.
Current Limit
The AP5100 has cycle-by-cycle current limiting
implementation. The voltage drop across the internal high-
side mosfet is sensed and compared with the internally set
current limit threshold. This voltage drop is sensed at
about 30ns after the HS turns on. When the peak inductor
current exceeds the set current limit threshold, current limit
protection is activated. During this time the feedback
voltage (VFB) drops down. When the voltage at the FB pin
reaches 0.4V, the internal oscillator shifts the frequency
from the normal operating frequency of 1.4MHz to a fold-
back frequency of 480kHz. The current limit is reduced to
70% of nominal current limit when the part is operating at
480kHz. This low Fold-back frequency prevents runaway
current.
Thermal Shutdown
The AP5100 has on-chip thermal protection that prevents
damage to the IC when the die temperature exceeds safe
margins. It implements a thermal sensing to monitor the
operating junction temperature of the IC. Once the die
temperature rises to approximately 140°C, the thermal
protection feature gets activated .The internal thermal
sense circuitry turns the IC off thus preventing the power
switch from damage.
A hysteresis in the thermal sense circuit allows the device
to cool down to approximately 120°C before the IC is
enabled again. This thermal hysteresis feature prevents
undesirable oscillations of the thermal protection circuit.
PC Board Layout
This is a high switching frequency converter. Hence
attention must be paid to the switching currents
interference in the layout. Switching current from one
power device to another can generate voltage transients
across the impedances of the interconnecting bond wires
and circuit traces. These interconnecting impedances
should be minimized by using wide, short printed circuit
traces. The input capacitor needs to be as close as
possible to the IN and GND pins. The external feedback
resistors should be placed next to the FB pin.
AP5100
Document number: DS32130 Rev. 2 - 2
8 of 10
www.diodes.com
November 2010
© Diodes Incorporated