UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
43 of 362
NXP Semiconductors
UM10208
Chapter 6: LPC2800 DC-DC converter
2.
General operation
The basic connections within the DC-DC converter are shown in
. Depicted are
two inductive DC-DC converters, which are used when the chip is operated from a battery
supply. These converters deliver 1.8 V and 3.3 V to the pins DCDC_V
DDO(1V8)
and
DCDC_V
DDO(3V3)
respectively. Note that externally required components are not shown in
.
When the chip is supplied from USB or other higher voltage source (in the range of 4.0 V
to 5.5 V), the DC-DC converters will be turned off and the two linear regulators will be
used instead, producing similar voltages on the DCDC_V
DDO
pins.
An internal bandgap reference and a Ring Oscillator are connected such that they are
powered whenever either the battery supply or the USB supply is receiving power. The
DC-DC controller checks the DC-DC converter output voltages when they are operating
and uses that information to adjust the converters to keep the output voltage in range.
During the start-up the DC-DC Controller uses the Ring Oscillator to control the switching
regulators. After start-up, software may switch the DC-DC clock to the 12 MHz crystal.
When operating from a battery supply, the output voltage of DCDC_V
DDO(3V3)
and
DCDC_V
DDO(1V8)
can be controlled by software. This is done via 3 adjustment bits in the
registers DCDCADJUST1 and DCDCADJUST2.
2.1 Local power
As previously mentioned, the internal bandgap reference and the Ring Oscillator are
powered whenever either the USB or battery supply is available. The power selected is
USB power (divided by 3) if it is available, followed by battery power if available.
2.2 Supply_OK
The output of the DC-DC converters or LDO regulators are monitored by comparators that
indicate when the supply is providing both 1.8 and 3.3 V power. This indication is used
internally by the DC-DC converter and is defined here so that it may be shown in the
power timing diagrams later in this section.
2.3 Battery connection in an application
below shows an example of how the DC-DC Converter may be connected in
an application that uses battery and/or USB power.