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| 型号： | MAX8672ETD+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 14/17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
Note: In order for BATT current to remain below 50nA
during BATT UVLO, V BST must fall below 0.5V. If V BST
BACKUP READY*
(BOOST AND LDO ENABLED)
V IN > 2.45V
is held up by another source during UVLO, or if V BST is
higher than 0.5V, BATT input current during BATT
UVLO is typically 500nA. Typically, V BST falls to 0V in
most situations. If minimum battery drain during BATT
UVLO is critical, then an external pulldown resistor con-
nected between BST and GND may be needed to dis-
V BATT < V BATT(UV)
BATT UVLO
(BOOST AND LDO OFF)
AND
V BST > 3.05V
AND
V BATT > V BATT(UV)
charge the BST output.
The 500nA BATT drain during UVLO is necessary when
V BST is > 0.5V because a comparator must be kept
active in order to detect the higher of V BATT or V BST .
This comparator switches the body diode of the internal
FET connecting these outputs to ensure that current flow
is blocked. When V BST falls to approximately 0.5V, the
comparator is shut off, and the FET body is connected to
block current flowing from BATT to BST.
* NOTE: BACKUP READY DOES NOT MEAN THAT THE BOOST AND LDO ARE OPERATING.
WHEN THE BST AND LDO OUTPUTS ARE ENABLED; THEY STILL ONLY OPERATE IF
NEEDED WHEN THE SYSTEM FAILS TO HOLD UP THE SUPPLIES.
Figure 5. Backup and BATT UVLO State Diagram
BATT Undervoltage Lockout
When the backup battery discharges to a programmed
threshold, V BATT(UV) , BATT UVLO is engaged. As a
result, the MAX8672 backup functions (BST and LDO)
shut down, and a small current (less than 50nA) is
drawn from BATT. During BATT UVLO, charge func-
tions still remain active to recharge the battery. Once
BATT UVLO occurs, the backup boost converter and
LDO do not reactivate until V BST rises above 3.05V and
V IN rises above 2.45V (typ). Even if BATT recovers, the
backup functions do not activate until a valid V IN and
V BST have been present. See the Backup and BATT
UVLO State Diagram (Figure 5).
The BATT UVLO threshold (V BATT(UV) ) is programmed by
connecting a resistor (R UV ) from UV to GND (Figure 4).
For NiMH cells, the UVLO threshold is typically pro-
grammed to 0.8V per cell. Once the UVLO threshold
value is determined, R UV is calculated from the following
equation:
BATT-BST Current Flow
The MAX8672 synchronous rectifier pMOS contains an
internal body diode connected between BATT and
BST. This diode switches to prevent undesired current
flow between these pins. Upon startup, the body diode
points to the greater of V BATT or V BST , until V BST rises
above 3.05V (at least once). Then the body diode
switches to point to BST. The body diode points from
BATT to BST until V BST falls below 2.2V. When this
occurs, the body diode switches to point to the greater
of V BATT or V BST .
If V BATT exceeds V BST by a few hundred millivolts or
more, the body diode is forward biased and current
flows from BATT to BST. This is the typical case for a
boost converter when the input exceeds the output.
When backing up, this typically is not a problem since it
is expected that battery current powers the system.
When not in backup mode (system power is up and is
pulling V BST over 3.05V), current can flow from BATT to
BST if V BATT exceeds V BST by enough to forward bias the
diode. With two NiMH cells, V BATT charges to 3.0V nomi-
nal (3.1V max), so with V BST pulled to more than 3.05V by
the system, there is not enough voltage difference to
cause significant current to flow from BATT to BST.
R UV =
V BATT(UV)
16 × 10 ? 6
14
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