AN1978 STMicroelectronics, AN1978 Datasheet - Page 5

• Manufacturer Part Number: AN1978
• Description: ST8004: SMART CARD INTERFACE
• Manufacturer: STMicroelectronics

without additional capacitors, even for a frequency higher than 26MHz.
Another recommendation about the CLK output is to keep the CLK wire far from the other signals
because inductive or capacitive effects could produce cross conduction on the transceiver lines.
Moreover it is better to shield the clock wire with a ground plain or track on the PCB.
4. STEP-UP CONVERTER AND POWER SUPPLY REGULATOR
The ST8004 can drive both 3V and 5V cards through the supply voltage selector 5/3V pin (pin 3) as
shown in Figure 6. If the 5/3V pin is to GND, the Vcc voltage is 3V and the internal Vcc regulator is
connected directly to the VDDP pin.
If the 5/3V pin is connected to VDD, the Vcc regulator is connected to the internal step-up converter, as
shown in Figure 6, in order to provide the high Vcc supply voltage (5V) even when the VDDP voltage is
lower than 5V.
Figure 6: Step-up converter block diagram.
The step-up converter is supplied by the VDDP pin; S1 and S2 pins are used to duplicate the supply
voltage through the 100nF pumping capacitor, while the charge pump output is connected to the VUP pin
that requires a 100nF storage capacitor to stabilize the voltage.
Due to the switching circuitry, a little noise is introduced so, in order to reduce it and improve the
efficiency of the step-up converter, the capacitors must be connected as close as possible to the pins as
shown in Figure 9 and are recommended with ESR lower than 50mΩ @ 100kHz such as MURATA
GRM31M7U1H104JA01B. Anyway, also capacitor with ESR up to 100mΩ @ 100kHz are enough to work
in specifications.
5/3V
100nF
3
EN2
P
VCC
ST8004
V
DDP
6
ON/OFF
REGULATOR
OUTPUT
100nF
VCC
S1
H
L
7
CONVERTER
STEP-UP
S2
5
17
P
V
V
GND
UP
CC
AN1978 - APPLICATION NOTE
100nF
5/14