Motorola 38C V100 Service Manual page 19

Level 3 Service Manual
6881036B20
The GCAP II is programmed to Boost Mode (5.6V) by PGM0 Pin G7 and PGM1 Pin
G8 both being tied to Ground. Once B+ is applied to GCAP II Pin K5, all the
appropriate voltages to supply the circuit are provided. These are:
• V1 - Programmed to 5.0V. V1 is at 2.775V at immediate power-on, but is
"boosted" to 5.0V through the switch mode power supply L901 / CR902 and
C913. See Figure 7 for basic operation. V1 supplies the DSC bus drivers, nega-
tive voltage regulators and Magic IC. V1 is created from GCAP II Pin A6 and
can be measured at C906.
Figure 7. V1 Boost Circuit
Basic circuit operation for the boost circuit is as follows: The LX signal (GCAP
II Pin B10) allows a path for B+ to charge the capacitor when the switch is on.
When the switch is off, the capacitor discharges through the inductor setting
up an electric field. The field then collapses setting up back EMF to charge the
capacitor, and so on. The back EMF created by the inductor is greater than B+,
with the +ve half of the cycle passing through the diode to charge a capacitor
that supplies the V_BOOST voltage. The frequency of the switching signal LX
determines the duty cycle of the output wave and therefore the resultant volt-
age. V_BOOST is fed back into the GCAP.
• V2 - Programmed to 2.775V is available whenever the radio is on and supplies
most of the logic side of the board. V2 is supplied from of GCAP II Pin J2 and
can be measured at either C939 or C941.
• V3 - Programmed to 2.003V to support the Whitecap IC, but does not support
the normal 2.75V logic output from the Whitecap IC. V3 originates from GCAP
II Pin B5 and can be measured at C909 or C910.
• VSIM1 - Used to power either 3V or 5V SIM cards. VSIM1 is dynamically set to
3V upon power up. If, however, the card cannot be read, the SIM card is pow-
ered down and attempted to be read at 5V. VSIM1 can be measured at C905
and is distributed from GCAP II Pin C6 (See Controller: SIM Card Interface on
Page 17).
• VREF - Programmed to 2.775V like V2. VREF provides a reference voltage for
the Magic IC and Deep Sleep circuit, is distributed from GCAP II Pin G9, and
can be measured at C919.
• -5V - Used to drive display and generate -10V. The -5V is also used for RF GSM
/ DCS selection signals through U151. Both voltages are derived from V1
through U903 and U904.
• SR_VCC (power cut circuit) - Used to buffer SRAM U702 voltage with a built
in soft reset within the unit's software. SR-VCC protects the user from acciden-
tal loss of power for up to 0.5 seconds, as might occur if the unit is dropped,
causing a slight battery contact bounce. SR_VCC allows the unit to appear to
be running normally while, internally, a reset is occurring. During this loss of
power the unit takes its power from RTC BATTERY+ which originates from
MOTOROLA CONFIDENTIAL - PROPRIETARY
November 20, 2000
B+ FR EAK
LX
Circuit Description
Output
000719-O
15