UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
45 of 362
NXP Semiconductors
UM10208
Chapter 6: LPC2800 DC-DC converter
3.
DC-DC converter timing
Several cases are given to illustrate operation of the DC-DC Converter block. The first
shows timing when the START signal is used to activate the chip when only battery power
is available. The second shows timing when USB power is connected when no battery
power is available. The third shows switching from battery power to USB power.
3.1 START and STOP from battery power
shows the timing of the DC-DC Converter while being started and stopped
when powered by a battery supply. Note that timing and voltage levels are not to scale.
A negative edge at the START input activates the DC-DC converter. When minimum
supply voltages are detected for DCDC_V
DDO(3V3)
and DCDC_V
DDO(1V8)
, SUPPLY_OK
becomes true. After about 1 ms (determined by a number of clock periods of the Ring
Oscillator), the internal active-low reset signal is de-asserted. Once started, additional
edges on the START pin have no effect on the DC-DC Converter.
The upper trace shows the effect on external circuitry as the DC-DC converter powers up,
as in the application example in
.
Fig 11. Example of DC-DC converter connections when the DC-DC converter is not used.
DCDC_V
DDO(3v3)
DCDC_V
DDO(1v8)
DCDC_V
DDO(3v3)
DCDC_V
BAT
DCDC_V
USB
STOP
START
DCDC_GND
DCDC_CLEAN
DCDC_LX1
DCDC_LX2
DCDC_V
SS1
DCDC_V
SS2
LPC288x
L18
L17
N18
N17
N16
P18
R18
P17
M16
L16
M18
M17
T18
External
power
source
External regulator
or DC-DC
converter
3.3V
1.8V
External regulator
or DC-DC
converter
1.8V Power Pins
3.3V Power Pins