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| 型号： | MAX1809ETI+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 7/17页 |
| 文件大小： | 0K |
| 描述： | IC DDR TERMINATION 28-TQFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | 转换器，DDR |
| 输入电压： | 3 V ~ 5.5 V |
| 输出数： | 1 |
| 输出电压： | 1.1 V ~ 5.5 V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-SSOP（0.154"，3.90mm 宽） |
| 供应商设备封装： | 16-QSOP |
| 包装： | 带卷 (TR) |
3A, 1MHz, DDR Memory Termination Supply
( 2 V DDQ )
V IN
10 ?
33 μ F
IN
MAX1809
LX
PGND
L
R DROOP
V OUT =
270 μ F
2V
15m ?
V CC
GND
2.2 μ F
SHDN
FB
V DDQ
(2.5V)
10k ?
1000pF
V SET
EXTREF
TOFF
R TOFF
REF
SS
0.01 μ F
1 μ F
10k ?
V SSQ
Figure 1. Typical Application Circuit
1000pF
FOR V IN = 5V: L = 1 μ H, R TOFF = 130k ?
FOR V IN = 3.3V: L = 0.68 μ H, R TOFF = 73.2k ?
feedback voltage exceeds the external reference voltage
(V EXTREF ) or the positive current limit is reached. When
the PMOS switch turns off, it remains off for the pro-
grammed off-time (t OFF ). To control the current under
short-circuit conditions, the PMOS switch remains off for
approximately 4 ? t OFF when V FB < V EXTREF / 4.
Synchronous Rectification
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.
Current Sourcing and Sinking
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
a second mode as a synchronous boost, taking power
from the output and returning it to the input.
Thermal Resistance
Junction-to-ambient thermal resistance, θ JA , is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1809 QFN package
has 1in 2 of copper area and a thermal resistance of
50 ° C/W with no forced airflow. The MAX1809 16-pin
QSOP evaluation kit has 0.5in 2 of copper area and a
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.
Shutdown
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 CC
for normal operation.
Power Dissipation
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:
P D(CAP) = C ? V IN 2 ? f SW
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