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LTC1159 Datasheet(PDF) 11 Page - Linear Technology

Part # LTC1159
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

LTC1159 Datasheet(HTML) 11 Page - Linear Technology

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11
LTC1159
LTC1159-3.3/LTC1159-5
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 reduced
MOSFET gate drive levels and an efficiency penalty of up to
10% at high input voltages.
2. EXTVCC Connected Directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage which has been boosted to greater
than 4.5V. This can be done either with the inductive boost
winding shown in Figure 5a or the capacitive charge pump
shown in Figure 5b. The charge pump has the advantage of
simple magnetics and generally provides the highest effi-
ciency at the expense of a slightly higher parts count.
VIN
P-DRIVE
LTC1159-3.3
P-GATE
N-CH
VIN
BAT85
P-CH
CIN
RSENSE
COUT
1
µF
VOUT
LTC1159 • F05a
N-GATE
P-GND
EXTVCC
L
1:1
+
+
+
4
3
2
1
Figure 5a. Inductive Boost Circuit for EXTVCC
Figure 5b. Capacitive Charge Pump for EXTVCC
VIN
P-DRIVE
LTC1159-3.3
P-GATE
N-CH
BAT85
0.22
µF
1
µF
BAT85
BAT85
VN2222LL
VIN
P-CH
CIN
RSENSE
COUT
VOUT
LTC1159 • F05b
N-GATE
P-GND
EXTVCC
L
+
+
+
APPLICATIO S I FOR ATIO


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