DS1390_09 Datasheet, PDF (19/26 Pages) Maxim Integrated Products – Low-Voltage SPI/3-Wire RTCs with Trickle Charger

Low-Voltage SPI/3-Wire RTCs with
Trickle Charger
TRICKLE-CHARGE REGISTER (8Fh WRITE, 0Fh READ)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
TCS3 TCS2 TCS1 TCS0
DS1
BIT 2
BIT 1
BIT 0
DS0 ROUT1 ROUT0
1 0F 16 SELECT
NOTE: ONLY 1010b ENABLES CHARGER
1 OF 2
SELECT
1 OF 3
SELECT
V_CC
TCS[3:0] = TRICKLE-CHARGE SELECT
DS[1:0] = DIODE SELECT
ROUT[1:0] = RESISTOR SELECT
R1
250Ω
R2
2kΩ
R3
4kΩ
V_BACKUP
Figure 8. DS1390–DS1394 Programmable Trickle Charger
Table 6. SPI Pin Function
MODE
Disable
Write
CPHA
X
0
CS
High
Low
SCLK
Input Disabled
CPOL* = 0, SCLK Rising;
CPOL = 1, SCLK Falling
Read
0
Low
CPOL = 0, SCLK Falling;
CPOL = 1, SCLK Rising
Write
1
Low
CPOL* = 1, SCLK Rising;
CPOL = 0, SCLK Falling
Read
1
Low
CPOL = 1, SCLK Falling;
CPOL = 0, SCLK Rising
*CPOL is the clock-polarity bit set in the control register of the host microprocessor.
**SDO remains at high-Z until 8 bits of data are ready to be shifted out during a read.
SDI
Input Disabled
Data Bit Latch
SDO
High-Z
High-Z
X
Next Data Bit Shift**
Data Bit Latch
High-Z
X
Next Data Bit Shift**
The user determines diode and resistor selection
according to the maximum current desired for battery
or super cap charging. The maximum charging current
can be calculated as illustrated in the following exam-
ple. Assume that a system power supply of 3.3V is
applied to V_CC and a super cap is connected to
V_BACKUP. Also, assume that the trickle charger has
been enabled with a diode and resistor R2 between
V_CC and V_BACKUP. The maximum current I_MAX would
therefore be calculated as follows:
I_MAX = (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) /
2kΩ ≈ 1.3mA
As the super cap changes, the voltage drop between
V_CC and V_BACKUP decreases and therefore the charge
current decreases.
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