NCN6000
http://onsemi.com
23
When the input voltage Vbat is lower than the
programmed CRD_VCC, the system operates under the
boost mode, providing the voltage regulation and current
limit to the smart card. In this mode, the external inductor,
typically 22 H, stores the energy to drive the +5.0 V card
supply from the external low voltage battery. The
oscillogram, Figure 18, depicts the DC−DC behavior under
these two modes of operation.
Beside the DC−DC converter, NMOS Q4 provides a low
impedance to ground during the Power Down sequence,
yielding the 250 s maximum switch time depicted in the
data sheet.
Figure 18. DC−DC Operating Modes
Step Down Mode
CRD_VCC 5 V Step Up Mode
CRD_VCC
I
L
Ibat
DC Operating Current @ CRD_VCC = 5.0 V
0
1
2
3
4
5
6
7
8
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Vbat (V)
Ibat_op (mA)
25°C
85°C
POWER_ON
Figure 19. Typical DC Operating Current
−25°C
When the input voltage Vbat is higher than the
programmed CRD_VCC, the system operates under a step
down mode, yielding the voltage regulation and current
limit identical to the boost mode. In this case, the built−in
structure turns Off Q1 and inverts the Q2 substrate bias to
control the current flowing to the load.These operations are
fully automatic and transparent for the end user.
The High and Low limits of the current flowing into the
external inductor L1 are sensed by the operational amplifier
U1 associated with the internal shunt R1. Since this shunt
resistor is located on the hot side of the inductor, the device
reads both the charge and discharge of the inductor,
providing a clean operation of the converter.
In order to optimize the DC−DC power conversion
efficiency, it is recommended to use external inductor with
R < 2.0 .
The output capacitor C1 stores the energy coming from the
converter and smooths the CRD_VCC voltage applied to the
external card. At this point, care must be observed, beside the
micro farad value, to select the right type of capacitor.
According to the capacitor’s manufacturers, the internal ESR
can range from a low 10 m to more than 3.0 , thus yielding
high losses during the DC−DC operation, depending upon the
technology used to build the capacitor.
The standard electrolytic capacitors have the low cost
advantage for a relative high micro farad value, but have
poor tolerance, high leakage current and high ESR.
The tantalum type brings much lower leakage current
together with high capacity value per volume, but cost can
be an issue and ESR is rarely better than 500 m.
The new ceramic type have a very low leakage together
with ESR in the 50 m range, but value above 10 F are
relatively rare. Moreover, depending upon the low cost
ceramic material used to build these capacitors, the thermal
coefficient can be very bad, as depicted in Figure 20. The
X7R type is highly recommended to achieve low voltage
ripple.
Figure 20. Typical Y7R Ceramic Type Value as a
Function of the Temperature.
100%
−25°C +25°C +85°C
15%
