Datasheet

11
LTC1149
LTC1149-3.3/LTC1149-5
APPLICATIO S I FOR ATIO
WUU U
Figure 5. High Efficiency Step-Down Regulator with V
OUT
> V
CC
regulator loop adapts to the current change and returns
V
OUT
to its steady state value. During this recovery time
V
OUT
can be monitored for overshoot or ringing which
would indicate a stability problem. The Pin 7 external
components shown in the Figure 1 circuit will prove
adequate compensation for most applications.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
OUT
, causing a rapid drop in V
OUT
. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25)(C
LOAD
).
Thus a 10µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
LTC1149 Adjustable Applications
When an output voltage other than 3.3V or 5V is required,
the LTC1149 adjustable version is used with an external
resistive divider from V
OUT
to V
FB
Pin 10. The regulated
voltage is determined:
V
OUT
= 1.25
) )
1 +
R2
R1
In applications where V
OUT
is greater than the LTC1149
internally regulated V
CC
voltage, R
SENSE
must be moved to
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes the Burst Mode
operation to be activated when the LTC1149 series would
normally be in continuous operation. The effect is most
pronounced with low values of R
SENSE
and can be
improved by operating at higher frequencies with lower
values of L. The output remains in regulation at all times.
Checking Transient Response
Switching regulators take several cycles to respond to a
step in DC (resistive) load current. When a load step
occurs, V
OUT
shifts by an amount equal to (∆I
LOAD
)(ESR),
where ESR is the effective series resistance of C
OUT
.
∆I
LOAD
also begins to charge or discharge C
OUT
until the
0.068µF
V
IN
CAP
PDRIVE
LTC1149
I
TH
C
T
PGATE
V
CC
V
CC
SHDN2
SENSE
–
SENSE
+
100pF
IRFZ34
0.047µF
1N4148
1N4148
V
IN
IRF9Z34
1N5819
R
SENSE
0.05Ω
150µF
50V
1µF
0V = NORMAL
>2V = SHUTDOWN
3300pF
C
T

200pF
1k
1149 F05
GNDS
NGATE
V
FB
1000pF
100µH
R2
215k
1%
R1
25k
1%
150µF
16V
OS-CON
LOAD
V
OUT
OUTPUT
GROUND
CONNECTION
V
OUT
= 1.25
()
1 +
R2
R1
VALUES SHOWN FOR V
OUT
= 12V
+
+
+
(V
IN
– V
OUT
) VOLTAGE (V)
0
C
OUT
(µF)
600
800
1000
4
1149 F04
400
200
0
1
2
3
5
L = 50µH
R
SENSE
= 0.02Ω
L = 25µH
R
SENSE
= 0.02Ω
L = 50µH
R
SENSE
= 0.05Ω
Figure 4. Minimum Suggested C
OUT