MAX1809EEE+T Maxim Integrated Products, MAX1809EEE+T Datasheet - Page 7

• Manufacturer Part Number: MAX1809EEE+T
• Description: IC DDR TERMINATION 16-QSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1809EEE+T

• Applications: Converter, DDR
• Voltage - Input: 3 ~ 5.5 V
• Number Of Outputs: 1
• Voltage - Output: 1.1 ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-QSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

feedback voltage exceeds the external reference voltage
(V
the PMOS switch turns off, it remains off for the pro-
grammed off-time (t
short-circuit conditions, the PMOS switch remains off for
approximately 4
In a stepdown regulator without synchronous rectification,
an external Schottky diode provides a path for current to
flow when the inductor is discharging. Replacing the
Schottky diode with a low-resistance NMOS synchro-
nous switch reduces conduction losses and improves
efficiency.
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay (approximately 50ns) after the PMOS
power switch turns off, thus preventing cross-conduc-
tion or “shoot-through.” In constant-off-time mode, the
synchronous-rectifier switch turns off just prior to the
PMOS power switch turning on. While both switches
are off, inductor current flows through the internal body
diode of the NMOS switch.
By operating in a constant-off-time, pseudo-fixed-fre-
quency mode, the MAX1809 can both source and sink
current. Depending on the output current requirement,
the circuit operates in two modes. In the first mode the
output draws current and the MAX1809 behaves as a
regular buck controller, sourcing current to the output
from the input supply rail. However, when the output is
supplied by another source, the MAX1809 operates in
Figure 1. Typical Application Circuit
EXTREF
) or the positive current limit is reached. When
3A, 1MHz, DDR Memory Termination Supply
✕
t
Current Sourcing and Sinking
OFF
_______________________________________________________________________________________
OFF
Synchronous Rectification
when V
). To control the current under
V
(2.5V)
V
DDQ
SSQ
10kΩ
10kΩ
FB
2.2µF
< V
10Ω
V
IN
EXTREF
1000pF
1000pF
V
33µF
SET
/ 4.
R
TOFF
IN
V
SHDN
EXTREF
TOFF
CC
MAX1809
PGND
GND
REF
SS
LX
FB
a second mode as a synchronous boost, taking power
from the output and returning it to the input.
Junction-to-ambient thermal resistance, θ
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1809 QFN package
has 1in
50°C/W with no forced airflow. The MAX1809 16-pin
QSOP evaluation kit has 0.5in
thermal resistance of 80°C/W with no forced airflow.
Airflow over the board significantly reduces the junction-
to-ambient thermal resistance. For heat sinking pur-
poses, it is essential to connect the exposed backside
pad of the QFN package to a large analog ground plane.
Drive SHDN to a logic-level low to place the MAX1809 in
low-power shutdown mode and reduce supply current to
less than 1µA. In shutdown, all circuitry and internal
MOSFETs turn off, so the LX node becomes high imped-
ance. Drive SHDN to a logic-level high or connect to V
for normal operation.
Power dissipation in the MAX1809 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to charging and discharging the
gate capacitance of the internal switches (i.e., switch-
ing losses) is approximately:
0.01µF
2
FOR V
FOR V
of copper area and a thermal resistance of
L
IN
IN
= 5V: L = 1µH, R
= 3.3V: L = 0.68µH, R
1µF
P
D(CAP)
R
DROOP
TOFF
= C
V
= 130kΩ
TOFF
OUT
270µF
2V
15mΩ
✕
=
= 73.2kΩ
( )
Thermal Resistance
V
V
DDQ
2
2
IN
Power Dissipation
of copper area and a
2 ✕
f
SW
Shutdown
JA
, is highly
CC
7