Datasheet

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APPLICATION INFORMATION
OPTIMIZING PERFORMANCE
TRANSFORMER DRIVE CIRCUIT
SELF-SYNCHRONIZATION
PCB Design
DCP02 Series
SBVS011K – MARCH 2000 – REVISED FEBRUARY 2008
The DCP01B , DCV01 , and DCP02 are three families
of miniature DC/DC converters providing an isolated
unregulated voltage output. All are fabricated using a
Optimum performance can only be achieved if the
CMOS/DMOS process with the DCP01B replacing
device is correctly supported. The very nature of a
the familiar DCP01 family that was fabricated from a
switching converter requires power to be instantly
bipolar process. The DCP02 is essentially an
available when it switches on. If the converter has
extension of the DCP01B family, providing a higher
DMOS switching transistors, the fast edges will create
power output with a significantly improved load
a high current demand on the input supply. This
regulation. The DCV01 is tested to a higher isolation
transient load placed on the input is supplied by the
voltage.
external input decoupling capacitor, thus maintaining
the input voltage. Therefore, the input supply does
not see this transient (this is an analogy to
high-speed digital circuits). The positioning of the
Transformer drive transistors have a characteristically
capacitor is critical and must be placed as close as
low value of transistor on resistance (R
DS
); thus, more
possible to the input pins and connected via a
power is transferred to the transformer. The
low-impedance path.
transformer drive circuit is limited by the base current
available to switch on the power transistors driving The optimum performance primarily depends on two
the transformer and the characteristic current gain factors:
(beta), resulting in a slower turn-on time.
1. Connection of the input and output circuits for
Consequently, more power is dissipated within the
minimal loss.
transistor, resulting in a lower overall efficiency,
2. The ability of the decoupling capacitors to
particularly at higher output load currents.
maintain the input and output voltages at a
constant level.
The input synchronizations facility (SYNC
IN
) allows
for easy synchronizing of multiple devices. If two to
The copper losses (resistance and inductance) can
eight devices (maximum) have their respective
be minimized by the use of mutual ground and power
SYNC
IN
pins connected together, then all devices will
planes (tracks) where possible. If that is not possible,
be synchronized.
use wide tracks to reduce the losses. If several
devices are being powered from a common power
Each device has its own onboard oscillator. This
source, a star-connected system for the track must
oscillator is generated by charging a capacitor from a
be deployed; devices must not be connected in
constant current and producing a ramp. When this
series, as this will cascade the resistive losses. The
ramp passes a threshold, an internal switch is
position of the decoupling capacitors is important.
activated that discharges the capacitor to a second
They must be as close to the devices as possible in
threshold before the cycle is repeated.
order to reduce losses. See the PCB Layout section
When several devices are connected together, all the
for more details.
internal capacitors are charged simultaneously.
When one device passes its threshold during the
charge cycle, it starts the discharge cycle. All the
other devices sense this falling voltage and, likewise,
initiate a discharge cycle so that all devices discharge
together. A subsequent charge cycle is only restarted
when the last device has finished its discharge cycle.
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