TD1509 Techcode Semiconductor, TD1509 Datasheet - Page 13

• Manufacturer Part Number: TD1509
• Description: 2A 150KHZ PWM Buck DC/DC Converter
• Manufacturer: Techcode Semiconductor

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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 3.8A and 4A. 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 TD1509 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 TD1509, and to the VOUT pins of
the TD1509. Current flows in the second loop
when the low side diode is on.
In PCB layout, minimizing the two loops area
reduces the noise of this circuit and improves
efficiency. A ground plane is recommended to
connect input capacitor, output capacitor, and
GND pin of the TD1509.
In the TD1509 buck regulator circuit, the two
major power dissipating components are the
TD1509 and output inductor. The total power
dissipation of converter circuit can be measured
December, 20, 2005.
2A 150KHZ PWM Buck DC/DC Converter
13
by input power minus output power.
P
The power dissipation of inductor can be
approximately calculated by output current and
DCR of inductor.
P
The junction to ambient temperature can be got
from power dissipation in the TD1509 and thermal
impedance from junction to ambient.
T
The maximum junction temperature of TD1509 is
145°C, which limits the maximum load current
capability. Please see the thermal de-rating
curves for the maximum load current of the
TD1509 under different ambient temperatures.
The thermal performance of the TD1509 is trongly
affected by the PCB layout. Extra care should be
taken by users during the design process to nsure
that the IC will operate under the recommended
environmental conditions.
Several layout tips are listed below for the best
electric and thermal performance.
1. Do not use thermal relief connection to the VIN
and the GND pin. Pour a maximized copper area
to the GND pin and the VIN pin to help thermal
dissipation.
2. Input capacitor should be connected to the VIN
pin and the GND pin as close as possible.
3. Make the current trace from VOUT pins to L to
the GND as short as possible.
4. Pour copper plane on all unused board area
and connect it to stable DC nodes, like VIN, GND,
or VOUT.
5. Keep sensitive signal traces such as trace
connecting FB pin away from the VOUT pins.
total _loss
inductor
(jun-amb)
_loss= I
=(P
= V
totalloss
IN
× I
O
IN
2
–P
× R
– V
inductorloss
Techcode Semiconductor Limited
inductor
O
× I
O
× 1.1
)× Θ
Datasheet
JA
TD1509