11
DS8241-03 January 2014
www.richtek.com
RT8241
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
FB1
OUT
FB
FB2
R
V
V
(1
)
R
=
× +
where V
FB
is as shown in Table 2.
Table 2. Feedback Voltage Selection
G0 State
G1 State
Feedback Voltage
0 0 V
FB
= 0.9V
0 1 V
FB
= 0.8V
1 0 V
FB
= 0.725V
1 1 V
FB
= 0.675V
Figure 2. Setting V
OUT
with a Resistor-Divider
Output Voltage Transition Operation
The digital input control pin Gx allows V
OUT
to transition
to both higher and lower values. For a downward transition,
the rapid change of Gx from high to low will suddenly cause
V
FB
to drop to a new internal V
REF
. At this time the LGATE
will drive high to turn on the low side MOSFET and draw
current from the output capacitor via the inductor. LGATE
will remain on until V
FB
falls to the new internal V
REF
, at
which point a normal UGATE switching cycle begins, as
shown in Figure 3. For a down transition, the low side
MOSFET remains on until V
FB
reaches the new internal
V
REF
. Thus, the negative inductor current will be increased.
If the negative current become large enough to trigger
NOCP, the low side MOSFET will be turned off to prevent
Figure 3. Output Voltage Down Transition
LGATE
PHASE
UGATE
FB
G0
G1
G0
G1
Q1
Q2
C
IN
V
IN
R
FB1
R
FB2
BOOT
V
OUT
C
OUT
For an upward transition (from lower to higher V
OUT
) as
shown in Figure 4, Gx changes from low to high and causes
V
FB
to rise to a new internal V
REF
. This quickly trips the
V
FB
comparator regardless of whether DEM is active or
not, generating an UGATE on-time and causing a
subsequent LGATE to be turned on. At the end of the
minimum off-time (400ns), if V
FB
is still below the new
internal V
REF
, another UGATE on-time will be started. This
sequence continues until the FB pin exceeds the new
internal V
REF
.
larger physical size and degraded load-transient response
(especially at low input voltage levels).
Output Voltage Setting (FB)
As Figure 2 shows, the output voltage can be adjusted
from 0.675V to 3.3V by setting the feedback resistors
R
FB1
and R
FB2
. Choose R
FB2
to be approximately 20k
Ω
,
and solve for R
FB1
using the equation :
large negative current from damaging the component.
Refer to the Negative Over Current Limit section for a full
description.
Figure 4. Output Voltage Up Transition
Gx
V
FB
V
OUT
UGATE
LGATE
V
REF
GND
Initial V
OUT
Final V
OUT
Initial V
REF
Final V
REF
GND
Gx
V
REF
Initial V
REF
Final V
REF
V
FB
UGATE
LGATE
Initial V
OUT
Final V
OUT
V
OUT
