7
DEMO MANUAL DC290
NO-DESIGN SWITCHER
For the purposes of these tests, the demonstration circuit
should be powered from a regulated DC bench supply, so
that variations on the DC input do not add errors to the
regulation measurements.
Another source of error may be the use of small spring-
clip leads when testing this circuit. Small spring-clip leads
are very convenient for small-signal bench testing and
voltage measurements, but should not be used with this
circuit. Soldered wire connections are required to prop-
erly ascertain the performance of the PC board.
Checking Transient Response
Switching regulators take several cycles to respond to a
step in DC (resistive) load current. When a load step
occurs, V
OUT
shifts by an amount equal to (
∆
I
LOAD
)(ESR),
where ESR is the effective series resistance of C
OUT
.
∆
I
LOAD
also begins to charge or discharge C
OUT
until the
regulator loop adapts to the current change and returns
V
OUT
to its steady-state value. During this recovery time,
V
OUT
can be monitored for overshoot or ringing, which
would indicate a stability problem. The external compo-
nents shown in Figure 1 will prove adequate for most
applications.
Long supply leads connected to V
IN
and GND may induce
ringing at V
IN
and V
OUT
resembling loop instability. This is
actually caused by the inductance of the long wires
resonating with the input ceramic capacitor. This phe-
nomenon is particularly pronounced when a ceramic
output capacitor is used. When using short leads to
connect V
IN
and GND is impractical, a 100
µ
F electrolytic
bulk capacitor can be soldered onto the board between V
IN
and GND. This should eliminate all ringing associated with
long V
IN
and GND leads. Space is provided on the PC board
for this purpose, as shown in Figure 6.
MANUFACTURER
DEVICE
PHONE
FAX
Central Semiconductor
Diodes
(516) 435-1110
(516) 435-1824
Coilcraft
Inductors
(847) 639-6400
(847) 639-1469
Coiltronics
Inductors
(561) 241-7876
(561) 241-9339
Dale
Inductors
(605) 665-9301
(605) 665-0817
International Rectifier
Diodes
(310) 322-3331
(310) 322-3332
ON Semiconductor
Diodes
(602) 244-6600
(602) 244-4015
Zetex
Diodes
(631) 543-7100
(631) 864-7630
Murata-Erie
Capacitors
(770) 436-1300
(770) 436-3030
Sprague
Capacitors
(207) 324-4140
(603) 244-1430
Sumida
Inductors
(847) 956-0667
(847) 956-0702
[81] 03-3607-5111
[81] 03-3607-5114
TDK
Inductors
(847) 803-6100
[81] 03-3278-5358
Figure 6. Space Provided for a Bulk Capacitor
Component Manufacturers
Table 2 is a partial list of manufacturers of components
that can be used in LTC1877 and LTC1878 applications.
Using components other than the ones supplied on the
demonstration board will require careful analysis to verify
that all component specifications are not exceeded.
Finally, recharacterizing the circuit for efficiency is
necessary.
JP3
V
IN
SYNC/MODE
RUN
GND
GND
LTC1877/LTC1878
High Efficiency Monolithic Synchronous
Step-Down Regulator
DEMO CIRCUIT DC290A
(408) 432-1900
V
OUT
1.5V
2.5V
3.3V
OPEN
GND
E1
E2
E3
E4
E5
E6
E7
JP1
JP2
JP3
JP3
SOLDER BULK
CAPACITOR HERE
Table 2. List of Alternative Component Manufacturers
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.