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| 型号： | MAX8672ETD+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 13/17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
Output-voltage ripple has two components: variations
in the charge stored in the output capacitor (C BST ) with
each BST pulse, and the voltage drop across the
capacitor’s ESR due to the current flow into and out of
the capacitor. The equations for approximating output-
voltage ripple are:
V RIPPLE = V RIPPLE ( C ) + V RIPPLE ( ESR )
V RIPPLE ( ESR ) = I PEAK × R ESR
LDO
For backup designs that require two different backup
voltages, the MAX8672 includes a small LDO, which is
powered from BST. This LDO can supply up to 20mA.
Generally, the output voltage is programmed to be just
below the minimum tolerance for the main supply.
When the main supply voltage drops below its speci-
fied level, the LDO begins regulating.
The LDO output voltage is adjustable from 1.5V to 3.05V
using external resistors (R4 and R5 in Figure 4). Since
2 ? ( V BST ? V BATT ) × C BST ?
V RIPPLE ( C ) =
1 ? L ?
? ? I PEAK
2
the FBL input bias current is 50nA (max), select feed-
back resistor R4 in the 100k Ω to 1M Ω range. After
choosing R4, calculate R5 as follows:
R 5 = R 4 ? LDO ? 1 ?
where I PEAK is the peak inductor current (see the Boost
Inductor Selection section). Since ESR is usually very
small in ceramic capacitors, the output ripple is typical-
ly dominated by V RIPPLE(C) .
Capacitance and ESR variation with temperature
should be considered for best performance in applica-
tions with wide operating-temperature ranges.
Boost Inductor Selection
The control scheme of the MAX8672 permits flexibility
in choosing an inductor. A 4.7μH inductor performs well
in most applications.
For maximum output current, choose the inductor value
so that the controller reaches the current limit before
the maximum on-time is reached:
? V ?
? V FBL ?
where V FBL = 1.25V.
Backup-Battery Bypass
Capacitor Selection
The MAX8672 boost converter draws 500mA short-term
inductor-charging current peaks from the battery when
the boost converter operates. Small coin cells that are
commonly used for backup often exhibit high output
impedance that varies over temperature. For this rea-
son, the backup battery must be bypassed with a high-
quality ceramic capacitor with X7R, X5R, or better
dielectric (C BATT , Figure 4). Typical values are between
10μF and 47μF. Note that high battery ripple can pre-
L <
V BATT × t ON(MAX )
I LIM
maturely trigger the UVLO comparator and shut down
the boost circuit before the battery is fully discharged.
If this is a concern with the selected battery, the UV
where t ON(MAX) is typically 5μs, and the current limit (I LIM )
is typically 500mA (see the Electrical Characteristics).
For larger inductor values, determine the peak inductor
current (I PEAK ) by:
threshold may be lowered, in addition to using a larger
battery bypass capacitance, to accommodate the
short-term battery-voltage dip due to ripple. See the
Battery Ripple vs. Temperature graph in the Typical
Operating Characteristics section.
I PEAK =
V BATT × t ON(MAX )
L
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