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參數(shù)資料
| 型號： | LTC1955IUH#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 2-CHANNEL POWER SUPPLY SUPPORT CKT, PQCC32 |
| 封裝： | 5 X 5 MM, 0.75 MM HEIGHT, MO-220WHHD, QFN-32 |
| 文件頁數(shù)： | 8/20頁 |
| 文件大?。?/td> | 243K |
| 代理商： | LTC1955IUH#TR |
LTC1955
16
1955fb
Using S.A.M. Cards
For applications using one or more installed S.A.M.
cards, the PRES A/PRES B pins for those sockets must
be grounded before operation of the card can occur
(assuming NC/NO is grounded). The PRES A/PRES B
pull-up current is designed for very low consumption,
but ultralow current can be achieved in shutdown by us-
ing a microcontroller output to pull down on the PRES
A/PRES B pins only when communication is necessary.
The fault detection circuitry will not allow a card socket
to be operated unless a card is detected.
Asynchronous Channel A Card Detection
Since the shift register is transparent when LD is held
low, DOUT is the same as D15. Recall from Table 1 that
D15 indicates the status of the card detection channel for
channel A. Thus, it is not necessary to perform an entire
read/write operation to determine the card detection
status of channel A. With LD low, DOUT can be used to
generate a real time card detection interrupt. This could be
useful for one S.A.M. card, one smart card application.
Inter Card Communication
Communication is possible directly from one card socket
to the other when both cards are selected at the same
time. This can be achieved by the following sequence of
actions.
1. Start with both cards off and deselected
2. Activate the supply of the slave card
3. Select the slave card only
4. Initiate a reset on the slave card
5. Deselect the slave card
6. Activate the supply of the master card
7. Select the master card only
8. Initiate a reset on the master card
9. Select both cards
Using the UNDERV Pin
The UNDERV pin can be used to add protection against a
supply undervoltage fault. By using two external program-
ming resistors, the undervoltage detection can be set to an
arbitrary level (Figure 7). To ensure that the smart cards
are properly shut down, there must be sufcient energy
available in the input bypass capacitor to run one or both
smart cards until the deactivation cycle begins. It can take
approximately 30μs from the detection of a fault until the
deactivation sequence begins. It is desirable to maintain
the VBATT supply at 2.7V or greater during this period.
Consider the following (worst-case) example:
1. The UNDERV pin is programmed to trip below 3.1V.
2. It is possible to have both cards activated at 5V and
drawing 60mA.
Since the output voltage is programmed to 5V, the charge
pump will be acting as a voltage doubler. With two cards
drawing 60mA each, the input current will be 2 (60mA
+ 60mA), or about 240mA. Allowing the VBATT supply to
droop from 3.1V to 2.7V during the 30μs timeout period,
the input capacitance would need to be at least:
240mA / [(3.1V – 2.7V) / 30μs] or 18μF.
Thermal Management
To minimize power dissipation, the LTC1955 will actively
decide whether to step up or down depending on the
required output voltages and available input voltage.
However, for optimum efciency, the LTC1955 should be
powered from a 3.3V supply.
If the input voltage is above 3.6V, and both cards are
drawing maximum current, there can be substantial power
dissipation in the LTC1955. If the junction temperature in-
creases above approximately 150°C, the thermal shutdown
circuitry will automatically deactivate both channels. To
reduce the maximum junction temperature, a good thermal
connection to the PC board is recommended.
Zero Shutdown Current
Although the LTC1955 is designed to have very low shut-
down current, it can still draw over a microampere on both
APPLICATIONS INFORMATION
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