TD1501 Techcode, TD1501 Datasheet - Page 13

• Manufacturer Part Number: TD1501
• Description: PWM Buck DC/DC Converter
• Manufacturer: Techcode

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Function Description(Cont.)
and so the output voltage ripple is mostly
independent of the ESR. The output voltage
ripple is estimated to be:
VRIPPLE ~= 1.4 * VIN * (fLC/fSW)^2
Where VRIPPLE is the output ripple voltage, VIN
is the input voltage, fLC is the resonant
frequency of the LC filter, fSW is the switching
frequency. In the case of tanatalum or low-
ESR electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency, and so the output ripple is
calculated as:
VRIPPLE ~= ∆I * RESR
Where VRIPPLE is the output voltage ripple, ∆I is
the inductor ripple current, and RESR is the
equivalent series resistance of the output
capacitors.
Output Rectifier Diode
The output rectifier diode supplies the current to the
inductor when the high-side switch is off. To reduce
losses due to the diode forward voltage and recovery
times, use a Schottky rectifier.
Table 1 provides the Schottky rectifier part numbers
based on the maximum input voltage and current rating.
Choose a rectifier who’s maximum reverse voltage rating
is greater than the maximum input voltage, and who’s
current rating is greater than the maximum load current.
Feedforward Capacitor (CFF)
For output voltages greater than approximately 8V, an
additional capacitor is required. The compensation
capacitor is typically between 100 pF and 33 nF, and is
wired in parallel with the output voltage setting resistor,
R2. It provides additional stability for high output
voltages, low input-output voltages, and/or very low ESR
December, 20, 2005. Techcode Semiconductor Limited www.techcodesemi.com
T
3A 150KHz PWM Buck DC/DC Converter
echcode
®
13
output capacitors, such as solid tantalum capacitors.
This capacitor type can be ceramic, plastic, silver mica,
etc.(Because of the unstable characteristics of ceramic
capacitors made with Z5U material, they are not
recommended.)
Note:In PCB layout. Reserved an area for CFF.
Over Current Protection (OCP)
The cycle by cycle current limit threshold is set between
4A and 5A. When the load current reaches the current
limit threshold, the cycle by cycle current limit circuit
turns off the high side switch immediately to terminate
the current duty cycle. The inductor current stops rising.
The cycle by cycle current limit protection directly limits
inductor peak current. The average inductor current is
also limited due to the limitation on peak inductor current.
When the cycle by cycle current limit circuit is triggered,
the output voltage drops as the duty cycle is decreasing.
Thermal Management and Layout
Consideration
In the TD1501 buck regulator circuit, high pulsing current
flows through two circuit loops. The first loop starts from
the input capacitors, to the VIN pin, to the VOUT pins, to
the filter inductor, to the output capacitor and load, and
then returns to the input capacitor through ground.
Current flows in the first loop when the high side switch is
on. The second loop starts from the inductor, to the
output capacitors and load, to the GND pin of the
TD1501, and to the VOUT pins of the TD1501. Current
flows in the second loop when the low side diode is on.
In PCB layout, minimizing the two loops area reduces the
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TD1501
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