Bay Linear, Inc
2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 606-5950, Fax: (925) 940-9556
www.baylinear.com
B1587
APPLICATION HINTS
The Bay Linear B1587 incorporates protection
against over-current faults, reversed load insertion,
over temperature operation, and positive and
negative transient voltage. However, the use of an
output capacitor is required in order to insure the
stability and the performances.
Stability
The output capacitor is part of the regulator’s
frequency compensation system.
Either a 220
µF
aluminum electrolytic capacitor or a 47
µF solid
tantalum capacitor between the output terminal and
ground guarantees stable operation for all operating
conditions.
However, in order to minimize overshoot and
undershoot, and therefore optimize the design,
please refer to the section ‘Ripple Rejection’.
Ripple Rejection
Ripple rejection can be improved by adding a
capacitor between the ADJ pin and ground. When
ADJ pin bypassing is used, the value of the output
capacitor required increases to its maximum (220
µF
for an aluminum electrolytic capacitor, or 47
µF for
a solid tantalum capacitor). If the ADJ pin is not
bypass, the value of the output capacitor can be
lowered to 100
µF for an electrolytic aluminum
capacitor or 15
µF for a solid tantalum capacitor.
However the value of the ADJ-bypass capacitor
should be chosen with respect to the following
equation:
C = 1 / ( 6.28 * FR * R1 )
Where C
= value of the capacitor in Farads
(select an equal or larger standard value),
FR
= ripple frequency in Hz,
R1
= value of resistor R1 in Ohms.
If an ADJ-bypass capacitor is use, the amplitude of
the output ripple will be independent of the output
voltage. If an ADJ-bypass capacitor is not used, the
output ripple will be proportional to the ratio of the
output voltage to the reference voltage:
M = VOUT / VREF
Where M
= multiplier for the ripple seen when
the ADJ pin is optimally bypassed.
VREF = Reference Voltage
Reducing parasitic resistance and inductance
One solution to minimize parasitic resistance and
inductance is to connect in parallel capacitors. This
arrangement will improve the transient response of
the power supply if your system requires rapidly
changing current load condition.
Thermal Consideration
Although the B1587 offers some limiting circuitry
for overload conditions, it is necessary not to exceed
the maximum junction temperature, and therefore to
be careful about thermal resistance. The heat flow
will follow the lowest resistance path, which is the
Junction-to-case thermal resistance.
In order to
insure the best thermal flow of the component, a
proper mounting is required. Note that the case of
the device is electrically connected to the output. In
case the case has to be electrically isolated, a
thermally conductive spacer can be used. However
do not forget to consider its contribution to thermal
resistance.
Assuming:
VIN = 10V, VOUT = 5V, IOUT = 4A, TA = 90°C, θCASE=
1
°C/W (no external heat sink, no wind)
Power dissipation under these conditions
PD = (VIN – VOUT) * IOUT = 15W
Junction Temperature
TJ = TA + PD * (θCASE+ θJC)
For the Control Section
TJ = 90°C + 15W*(1°C/W + 0.6°C/W) = 114°C
114
°C < T
JUNCTION MAX for the control section.
For the Power Section
TJ = 90°C + 15W*(1°C/W + 1.6°C/W) = 129°C
129
°C < T
JUNCTION MAX for the power transistor.
In both case reliable operation is insured by adequate
junction temperature.
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