LTC1159 Linear Technology, LTC1159 Datasheet - Page 10

• Manufacturer Part Number: LTC1159
• Description: High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1159/LTC1159-3.3/LTC1159-5
pronounced with low values of R
improved by operating at higher frequencies with lower
values of L. The output remains in regulation at all times.
Load Transient Response
Switching regulators take several cycles to respond to a
step in DC (resistive) load current. When a load step
occurs, V
where ESR is the effective series resistance of C
regulator loop adapts to the current change and returns
V
V
would indicate a stability problem. The I
components shown in the Figure 1 circuit will provide
adequate compensation for most applications.
A second, more severe transient is caused by switching in
loads with large (>1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
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
Thus a 10 F capacitor would require a 250 s rise time,
limiting the charging current to about 200mA.
10
OUT
OUT
I
LOAD
to its steady state value. During this recovery time
can be monitored for overshoot or ringing which
OUT
also begins to charge or discharge C
1000
OUT
, causing a rapid drop in V
800
600
400
200
0
Figure 4. Minimum Suggested C
0
shifts by an amount equal to I
R
SENSE
1
U U
L = 50 H
(V
= 0.05
IN
– V
L = 50 H
R
L = 25 H
R
2
OUT
SENSE
SENSE
) VOLTAGE (V)
= 0.02
= 0.02
3
OUT
W
4
LTC1159 • TPC04
SENSE
. No regulator can
OUT
5
and can be
OUT
LOAD
TH
U
until the
external
LOAD
ESR,
OUT
.
.
Line Transient Response
The LTC1159 has better than 60dB line rejection and is
generally impervious to large positive or negative line
voltage transients. However, one rarely occurring condi-
tion can cause the output voltage to overshoot if the proper
precautions are not observed. This condition is a negative
V
positive transition of greater than 0.5V/ s slew rate.
The reason this condition rarely occurs is because it takes
tens of amps to slew the regulator input capacitor at this
rate! The solution is to add a diode between the cap and V
pins of the LTC1159 as shown in several of the typical
application circuits. If you think your system could have
this problem, add the diode. Note that in surface mount
applications it can be combined with the P-gate diode by
using a low cost common cathode dual diode.
EXTV
The LTC1159 contains an internal PNP switch connected
between the EXTV
supplies the V
in voltage than the 4.5V internal regulator. This allows the
MOSFET driver and control power to be derived from the
output during normal operation and from the internal
regulator when the output is out of regulation (start-up,
short circuit).
Significant efficiency gains can be realized by powering V
from the output, since the V
driver and control currents will be scaled by a factor of
(Duty Cycle)/(Efficiency). For 5V regulators this simply
means connecting the EXTV
ever, for 3.3V and other low voltage regulators, additional
circuitry is required to derive V
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
IN
only from the internal 4.5V regulator resulting in re-
duced MOSFET gate drive levels and an efficiency pen-
alty of up to 10% at high input voltages.
transition of several volts followed within 100 s by a
CC
CC
Pin Connection
Left Open. This will cause V
CC
CC
power whenever the EXTV
:
CC
and V
CC
CC
IN
pins. The switch closes and
CC
current resulting from the
pin directly to V
power from the output.
CC
CC
to be powered
pin is higher
OUT
. How-
CC
IN