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| 型号： | MAX8672ETD+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 12/17页 |
| 文件大小： | 0K |
| 描述： | IC MANAGE NIMH BACKUP 14-TDFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 功能： | 备份管理 |
| 电池化学： | 镍金属氢化物 |
| 电源电压： | 2.7 V ~ 5.5 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-WFDFN 裸露焊盘 |
| 供应商设备封装： | 14-TDFN-EP（3x3） |
| 包装： | 带卷 (TR) |
Complete Backup Management
IC for NiMH Batteries
DR charging is allowed only when the THRM tempera-
ture is within hot and cold limits. The rising battery-volt-
age threshold for DR (V BATT(DR) ) is given by the
following equation:
V BATT ( DR ) = 0 . 667 × V BATT ( CHG )
The DR output voltage is:
V DR = 0 . 816 × V BATT ( CHG )
Thermistor Monitor
The thermistor monitor suspends standard charging
(and pauses the standard charge timer) when the ther-
mistor temperature moves above +45°C or below -1°C.
The thermistor must be an NTC type with a nominal
+25°C resistance of 100k Ω .
The temperature trip thresholds are adjusted by adding
external resistors in series and in parallel with the ther-
mistor. For the specified thermistor, the resistors values
are shown in Table 1.
Table 1. Series/Parallel Resistors for
Different Thermistor Thresholds ( β )
below the minimum tolerance for the main supply.
When the main supply voltage drops below its speci-
fied level, the step-up converter begins regulating as
long as the load is 20mA or less. The MAX8672 blocks
reverse current flow if V BST is higher than V BATT .
The MAX8672 typical application expects that a valid
system voltage is connected to BST and IN before
backup operations are required. The boost DC-DC
converter is able to supply a system load (up to 20mA)
when the main power source falls below the BST preset
voltage, but the IC cannot start up the BST output with
just the backup battery alone. BST must initially be
powered by the external system in order for the boost
converter to start. Then, if the system voltage falls
below the BST preset voltage, the boost converter can
supply the load. If necessary, this limitation can be
overcome for some applications by connecting a diode
from IN to BST, so that BST is immediately powered
from IN.
When V BATT is less than V BST , and V BST is not exter-
nally pulled above 3.05V by the main system supply,
the boost converter runs as needed to maintain V BST at
3.05V. If, during normal active/charging mode opera-
SERIES R
(k Ω )
0
7.5
13.7
18.7
18.8
22.7
23
PARALLEL R
(M Ω )
None
None
None
None
6.8
None
5.6
HOT TEMP
( O C)
45
50
55
60
59.9
65
65
COLD TEMP
( O C)
-1
-0.6
-0.3
0
-1
0
-1
tion, V BATT rises above the main system voltage that is
connected to BST, current may flow from the battery to
the main system supply, even though no backup oper-
ation is expected. For example, in a 2-cell system, if
V BATT is 3.2V and the system supply is holding BST at
3.1V, then the backup battery drains into the system
supply. The boost synchronous rectifier pMOS contains
a body diode that is switched to prevent unwanted cur-
rent flow (see the BATT-BST Current Flow section).
Since the normal maximum charge limit (V BATT(CHG) )
for 2 NiMH cells is usually set to 3.0V (for a 3.1V max),
8.6 1.7 50
Note: With 100k Ω thermistors at +25°C, β = 3977.
-5
and a 3.3V system supply less a 5% tolerance is
3.135V, V BATT does not exceed V BST during normal
system operation, resulting in no backup current flow.
Charge Timer
The MAX8672 includes a charge timer that is program-
mable from 2min to 2000min. Timer duration is pro-
grammed by a capacitor, C CT , connected from CT to
GND (Figure 4). The charge-timer duration (t CHG ) is
determined by the equation:
t CHG (minutes) = 10195 × C CT ( μ F )
Boost DC-DC Converter
The MAX8672 contains a low-current synchronous-rec-
tified boost converter that can supply up to 20mA. The
boost converter ’s preset output voltage is 3.05V,
intended for backing up a 3.3V supply. Preset output
voltages can be obtained from the factory on request.
Generally, the output voltage is programmed to be just
However, for other BATT or BST voltages where unwant-
ed backup current flow may occur, it can be prevented
by connecting a diode in series with the boost inductor
to reduce the voltage at BST. The diode may be a
Schottky or silicon signal diode, depending on how
much voltage needs to be dropped.
Boost Output Capacitor Selection
Choose output capacitors to supply output peak cur-
rents with acceptable voltage ripple. Low equivalent
series resistance (ESR) capacitors are recommended.
Ceramic capacitors have the lowest ESR, but low-ESR
tantalum or polymer capacitors offer a good balance
between cost and performance.
12
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