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LTC1159

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1159

LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

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 10µF 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

VIN

VIN

P-GATE

P-DRIVE

LTC1159-3.3

N-GATE

P-GND

EXTVCC

+

CIN

BAT85

P-CH

L

1:1 •

+

1µF

1 RSENSE 2

•

3

4

N-CH

+

COUT

VOUT

LTC1159 • F05a

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

VIN

P-GATE

P-DRIVE

LTC1159-3.3

N-GATE

P-GND

EXTVCC +

+

P-CH

CIN

L

RSENSE

N-CH

VN2222LL

BAT85

+

BAT85 0.22µF

BAT85

1µF

LTC1159 • F05b

VOUT

COUT

Figure 5a. Inductive Boost Circuit for EXTVCC

Figure 5b. Capacitive Charge Pump for EXTVCC

11

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