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CDMA-2X-11004-P1
Objective Specification
System description
Page 21 of 79
Switching regulator
The characteristics of the switching regulator connected to VCC pins should meet the following
requirements:
Power capability: the switching regulator with its output circuit must be capable of providing a
voltage value to the VCC pins within the specified operating range and must be capable of
delivering greater than 1.2 Amps for safe design margin
Low output ripple: the switching regulator together with its output circuit must be capable of
providing a clean (low noise) VCC voltage profile
High switching frequency: for best performance and for smaller applications select a switching
frequency
≥
600 kHz (since L-C output filter is typically smaller for high switching frequency). The
use of a switching regulator with a variable switching frequency or with a switching frequency
lower than 600 kHz must be carefully evaluated since this can produce noise in the VCC
voltage profile. An additional L-C low-pass filter between the switching regulator output to VCC
supply pins can mitigate the ripple on VCC, but adds extra voltage drop due to resistive losses
on series inductors
PWM mode operation: select preferably regulators with Pulse Width Modulation (PWM) mode.
While in active mode Pulse Frequency Modulation (PFM) mode and PFM/PWM mode transitions
must be avoided to reduce the noise on the VCC voltage profile. Switching regulators able to
switch between low ripple PWM mode and high efficiency burst or PFM mode can be used,
provided the mode transition from idle mode (current consumption approximately 2 mA) to
active mode (current consumption approximately 100 mA): it is permissible to use a regulator
that switches from the PWM mode to the burst or PFM mode at an appropriate current
threshold (e.g. 60 mA)
Output voltage slope: ( not necessary for CDMA solution, ok to delete-RJC) the use of the soft
start function provided by some voltage regulator must be carefully evaluated, since the
voltage at the VCC pins must ramp from 2.5 V to 3.2 V within 1 ms to allow a proper switch-on of
the module
Figure 4 and the components listed in Table 4 show an example of a high reliability power supply
circuit, where the module VCC is supplied by a step-down switching regulator capable of
delivering 2.5 A current pulses with low output ripple and with fixed switching frequency in PWM
mode operation greater than 1 MHz. The use of a switching regulator is suggested when the
difference from the available supply rail to the VCC value is high: switching regulators provide good
efficiency transforming a 12 V supply to the typical 3.8 V value of the VCC supply.
u-blox C200
12V
C6
R3
C5
R2
C3
C2
C1
R1
VIN
RUN
VC
RT
PG
SYNC
BD
BOOST
SW
FB
GND
6
7
10
9
5
C7
1
2
3
8
11
4
C8
C9
L2
D1
R4
R5
L1
C4
U1
VCC
GND
Figure 4: Suggested schematic design for the VCC voltage supply application circuit using a step-down regulator
