User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 45
Issue 1 07/99
The main portion of the power for the PAs during Tx burst are supplied by
the output capacitors, C109 – C113, the switcher current being limited to
1.6 A in burst, and 1.0 A in between bursts. The capacitors are recharged
in between the Tx bursts, with less current to obtain a better converter
efficiency. The output voltage is selected as low as possible, but still
sufficiently high, in order to prevent Vdc_out from dropping below the
CCONT regulation level of 3.1V, due to the current drawn during Tx burst.
U/D GND
LX Vout
FB
RLimH
RLimL
SHDN
ILimSel
MAD2PR1
MAD2PR1
MAD2PR1
TXP
VCON_2
VCON_1
R108
R109
V1016u8H
C115
Vb Vdc_out
L103
C109 C111
C112
C113
C110
R102R105R112
R114
V108b
V108a
470u
470u
470u
470u
3x
L102
Battery
R132R131
R113
R134
R 143
R128
R144
MAD2PR1
V105
V109a
LOW_BATT
R141
C154
V109b
Vdc_out R135
Vdc_out
Figure 11. DC/DC Converter
The 5 x 470uF output capacitors has been chosen to have low ESR to
achieve a compromise between output ripple, drop during Tx burst,
conversion ratio and efficiency.
The TXP from MAD2PR1 selects between two current limits, determined
by R108 and R109. TXP is active during the TX burst thus commanding a
higher current through the converter without saturating the coil L102. In
between the Tx bursts, when in call, the switcher current is lowered for
efficiency reasons, but still insuring full re–charge of the output capacitors
at the succeeding Tx burst.
The switching frequency is app. 600 KHz and in order to avoid emission in
neighboring channels, due to the PA being supplied from Vdc_out a ferrite
bead, L103 is inserted to attenuate the switching frequency.