SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 103 of 217
2.2.1.3
Guidelines for VCC supply circuit design using a Low Drop-Out (LDO) linear regulator
The use of a linear regulator is suggested when the difference from the available supply rail and the
VCC
value is
low: linear regulators provide high efficiency when transforming a 5 V supply to a voltage value within the
module
VCC
normal operating range.
The characteristics of the LDO linear regulator connected to the
VCC
pins should meet the following
prerequisites to comply with the module
VCC
requirements summarized in Table 6:
Power capabilities
: the LDO linear regulator with its output circuit must be capable of providing a proper
voltage value to the
VCC
pins and of delivering to
VCC
pins the specified maximum peak / pulse current
with 1/8 duty cycle (see the
SARA-G3
[1] or the
SARA-U2 series Data Sheet
[2]).
Power dissipation
: the power handling capability of the LDO linear regulator must be checked to limit its
junction temperature to the maximum rated operating range (i.e. check the voltage drop from the max input
voltage to the min output voltage to evaluate the power dissipation of the regulator).
Output voltage slope
: the use of the soft start function provided by some voltage regulator should be
carefully evaluated, since the
VCC
pins voltage must ramp from 2.5 V to 3.2 V in less than 1 ms to switch on
the SARA-U2 modules or in less than 4 ms to switch on the SARA-G3 modules by applying
VCC
supply, that
otherwise can be switched on by forcing a low level on the
RESET_N
pin during the
VCC
rising edge and
then releasing the
RESET_N
pin when the
VCC
supply voltage stabilizes at its proper nominal value.
Figure 43 and the components listed in Table 26 show an example of a high reliability power supply circuit,
where the
VCC
module supply is provided by an LDO linear regulator capable of delivering the specified highest
peak / pulse current, with proper power handling capability. The regulator described in this example supports a
wide input voltage range, and it includes internal circuitry for reverse battery protection, current limiting, thermal
limiting and reverse current protection.
It is recommended to configure the LDO linear regulator to generate a voltage supply value slightly below the
maximum limit of the module
VCC
normal operating range (e.g. ~4.1 V as in the circuit described in Figure 43
and Table 26). This reduces the power on the linear regulator and improves the whole thermal design of the
supply circuit.
5V
C1
IN
OUT
ADJ
GND
1
2
4
5
3
C2
R1
R2
U1
SHDN
SARA-G3 / SARA-U2
52
VCC
53
VCC
51
VCC
GND
Figure 43: Suggested schematic design for the VCC voltage supply application circuit using an LDO linear regulator
Reference
Description
Part Number - Manufacturer
C1, C2
10 µF Capacitor Ceramic X5R 0603 20% 6.3 V
GRM188R60J106ME47 - Murata
R1
9.1 k
Resistor 0402 5% 0.1 W
RC0402JR-079K1L - Yageo Phycomp
R2
3.9 k
Resistor 0402 5% 0.1 W
RC0402JR-073K9L - Yageo Phycomp
U1
LDO Linear Regulator ADJ 3.0 A
LT1764AEQ#PBF - Linear Technology
Table 26: Suggested components for VCC voltage supply application circuit using an LDO linear regulator
