PRELIMINARY
CM3202-02
Application Info (cont’d)
current. The CM3202-02 is designed for optimal
operation from a nominal 3.3VDC bus, but can work
with VIN up to 5V. When operating at higher VIN
voltages, attention must be given to the increased
package power dissipation and proportionally
increased heat generation. Limited by the package
thermal resistance, the maximum output current of the
device at higher VIN cannot exceed the limit imposed
by the maximum power dissipation value.
V
REF
is typically routed to inputs with high impedance,
such as a comparator, with little current draw. An
adequate V
REF
can be created with a simple voltage
divider of precision, matched resistors from V
DDQ
to
ground. A small ceramic bypass capacitor can also be
added for improved noise performance.
Input and Output Capacitors
The CM3202-02 requires that at least a 220
μ
F
electrolytic capacitor be located near the VIN pin for
stability and to maintain the input bus voltage during
load transients. An additional 4.7
μ
F ceramic capacitor
between the VIN and GND, located as close as
possible to those pins, is recommended to ensure
stability.
At a minimum, a 220μF electrolytic capacitor is
recommended for the V
DDQ
output. An additional
4.7μF ceramic capacitor between the V
DDQ
and GND,
located very close to those pins, is recommended.
At a minimum, a 220
μ
F electrolytic capacitor is
recommended for the V
TT
output. This capacitor
should have low ESR to achieve best output transient
response. SP or OSCON capacitors provide low ESR
at high frequency, and thus are a good choice. In
addition, place a 4.7
μ
F ceramic capacitor between the
V
TT
pin and GND, located very close to those pins. The
total ESR must be low enough to keep the transient
within the V
TT
window of 40mV during the transition for
source to sink. An average current step of ±0.5A
requires:
40mV
-
ESR
<
-------------- = 40mΩ
1A
1.) C
in
=C
ddq
=C
tt
=220
μ
F/4.7
μ
F for the full temperature
range of –40 to +85°C.
2.) C
in
=C
ddq
=C
tt
=100
μ
F/2.2
μ
F for the temperature
range of –25 to +85°C.
Adjusting VDDQ Output Voltage
The CM3202-02 internal bandgap reference is set at
1.25V. The V
DDQ
voltage is adjustable by using a
resistor divider, R1 and R2:
R1
+
R2
-
V
DDQ
=
V
ADJ
×
--------------------
R2
where V
ADJ
= 1.25V. The recommended divider value
is R
1
=R
2
=10k
Ω
for DDR-1 application, and
R1=4.42k
Ω
, R2=10k
Ω
for DDR-2 application
(V
DDQ
=1.8V, V
TT
=0.9V).
Shutdown
ADJSD also serves as a shutdown pin. When this is
pulled high (SHDN_H), both the VDDQ and the VTT
outputs tri-state and could sink/source less than 10μA.
During shutdown, the quiescent current is reduced to
less than 0.5mA, independent of output load.
It is recommended that a low leakage Schottky diode
be placed between the ADJSD Pin and an external
shutdown signal to prevent interference with the ADJ
pin’s normal operation. When the diode anode is pulled
low, or left open, the CM3202-02 is again enabled.
Current Limit and Over-temperature Protection
The CM3202-02 features internal current limiting with
thermal protection. During normal operation, V
DDQ
limits the output current to approximately 2A and V
TT
limits the output current to approximately ±2A. When
V
TT
is current limiting into a hard short circuit, the
output current folds back to a lower level (~1A) until the
over-current condition ends. While current limiting is
designed to prevent gross device failure, care should
be taken not to exceed the power dissipation ratings of
the package. If the junction temperature of the device
exceeds 170°C (typical), the thermal protection
circuitry triggers and tri-states both VDDQ and VTT
outputs. Once the junction temperature has cooled to
below about 120°C the CM3202-02 returns to normal
operation.
Both outputs will remain stable and in regulation even
during light or no load conditions. The general
recommendation for circuit stability for the CM3202-02
requires the following:
© 2007 California Micro Devices Corp. All rights reserved.
05/25/07
490 N. McCarthy Blvd., Milpitas, CA 95035-5112
●
Tel: 408.263.3214
●
Fax: 408.263.7846
●
www.cmd.com
9
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
