LT3667
30
3667fb
For more information www.linear.com/LT3667
TYPICAL APPLICATIONS
This application allows a small input current to support
a high current pulsed load. The switching regulator is
supplied by the LDO2 at OUT2, which is programmed to
limit its current to 3.5mA. PG2 serves as “READY” signal
to tell a controller (not shown) that C6 is charged to 17V,
the regulation voltage of LDO2. It can then turn on a load
drawing high current out of the switching regulator.
Since LDO2 can only supply 3.5mA, this quickly discharges
C6 and decreases V
OUT2
(=V
IN1
). The switching regulator
will maintain its programmed output voltage until V
IN1
drops below the undervoltage lockout threshold of 5.5V
set by R3 and R4.
Pulsed Power Supply for 4mA to 20mA Current Loops.
OUT2 Supplies the Switching Regulator, Which Is Kept Off at Lower Voltages by UVLO1
EN2/ILIM2 EN3/ILIM3
GND
EN
LT3667
BOOST
LDO2 Input
SW
DA
FB1
IN3
PG2
BD
OUT3
FB3
3667 TA06a
ON OFF
UVLO1
IN1
RT
OUT2
f = 600kHz
FB2
3.5mA Current Limit
C1-C5: X5R OR X7R
L1: CDRH4D22/HP
IN2
C1
1µF
C4
2.2µF
C2
0.22µF
D1
DFLS160
C8
22pF
L1
22µH
R1
442k
3.3V
100mA
READY
1.8V
100mA
R2
249k
R10
150k
R7
90.9k
R8
3010k
R9
150k
R5
200k
R6
158k
R
T
174k
R3
4.5M
Threshold
5.5V
Regulation
Voltage 17V
PG2 Indicates
when C6 is
Charged
Big Capacitor
Here to
Accumulate
Energy
R4
1M
V
IN
18V TO 45V
TRANSIENTS
DOWN TO –28V
C3
22µF
C6
1000µF
C7
47nF
C5
10µF
LDO2 Output
BUCK Input
0
1
2
3
4
5
6
8
10
12
14
16
18
0
1
2
3
4
0 1 2 3 4
TIME (SECONDS)
3667 TA06b
5
0
20
40
60
80
100
I
IN2
(mA)I
LOAD
(mA) V
OUT2
(V)V
PG2
(V)
I
IN2
Limited by Current
Limit of LDO2
V
OUT2
Rises as C6 is Charged
by the Constant Current I
OUT2
C6 is Quickly Discharged
by High Load Current
PG2 Signals That
V
OUT2
is High Enough
Controller Decides
to Activate Load
I
IN2
Drops as V
OUT2
Reaches Programmed Value
