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LTC1159CN-5 Datasheet(PDF) 10 Page - Linear Technology

Part # LTC1159CN-5
Description  High Efficiency Synchronous Step-Down Switching Regulators
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1159CN-5 Datasheet(HTML) 10 Page - Linear Technology

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LTC1159/LTC1159-3.3/LTC1159-5
APPLICATIO S I FOR ATIO
Figure 4. Minimum Suggested COUT
(VIN – VOUT) VOLTAGE (V)
0
600
800
1000
4
LTC1159 • TPC04
400
200
0
1
2
3
5
L = 50
µH
RSENSE = 0.02Ω
L = 25
µH
RSENSE = 0.02Ω
L = 50
µH
RSENSE = 0.05Ω
pronounced with low values of RSENSE and can be
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, VOUT shifts by an amount equal to ∆ILOAD × ESR,
where ESR is the effective series resistance of COUT.
∆ILOAD also begins to charge or discharge COUT until the
regulator loop adapts to the current change and returns
VOUT to its steady state value. During this recovery time
VOUT can be monitored for overshoot or ringing which
would indicate a stability problem. The ITH external
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
µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. 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
× CLOAD.
Thus a 10
µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
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
VIN transition of several volts followed within 100µs by a
positive transition of greater than 0.5V/
µs 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 VIN
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.
EXTVCC Pin Connection
The LTC1159 contains an internal PNP switch connected
between the EXTVCC and VCC pins. The switch closes and
supplies the VCC power whenever the EXTVCC pin is higher
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 VCC
from the output, since the VIN current resulting from the
driver and control currents will be scaled by a factor of
(Duty Cycle)/(Efficiency). For 5V regulators this simply
means connecting the EXTVCC pin directly to VOUT. How-
ever, for 3.3V and other low voltage regulators, additional
circuitry is required to derive VCC power from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC Left Open. This will cause VCC to be powered
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.


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