AN1978 STMicroelectronics, AN1978 Datasheet - Page 6

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

AN1978 - APPLICATION NOTE
A better noise performance could be achieved connecting the V
V
obtain a higher current from the charge pump when pulsed load is applied on Vcc.
In the typical applications a 100nF filter capacitor is connected to the Vcc output, near the ST8004 pins
and another 100nF is connected, between C1 and C5, near the card slot as shown in Figure 7 and
Figure 9. In order to reduce the noise presence and avoid cross conduction effects the wire length
between ST8004 and the card should be as short as possible (a wire less than 5cm-long is suggested).
In this way the Vcc spikes are much lower than 350mVpp, even when an impulsive load up to 65mA is
applied, as specified in the NDS requirements.
Figure 7: Smart Card connection.
A typical Vcc output waveform, with an impulsive 65mA load applied (worst condition at f=55KHz;
D.C.=50%) is shown in Figure 8 where the measured ripple is lower than 240mVpp; the Vcc pin will be
able to handle fast current transient generated by the smart card and remain between 4.60V and 5.30V
for Class-A and between 2.7V and 3.3V for Class-B (including ripple).
6/14
DDP
CGND connections
between ST8004 and
Card should be short
and straight.
instead of GND. Moreover, a 200nF capacitor should be connected to V
R1
100K
Ω
To V
DD
CGND
PRES
AUX2
AUX1
I/O
10
11
12
13
14
100nF
CARD SLOT
ST8004
C5
C6
C7
C8
C10
100nF
C1
C9
C2
C3
C4
UP
19
18
17
16
15
K1
K2
(100nF) capacitor between V
CMDVCC
V
Vcc
RST
CLK
THSEL
C3 (CLK) wire
should be routed far
from the other card
contacts.
J1
DDP
into GND in order to
UP
and