Linear Technology DC193 Demo Manual page 4

DEMO MANUAL DC193
NO DESIGN SWITCHER
OPERATIO
The circuit shown in Figure 2 operates from input voltages
from 2.5V to 6V. The output voltage is either 2.5V, 3.3V or
adjustable between 2.5V and 5V, depending upon the
position of jumper J1.
This demonstration unit is intended for the evaluation of
the LTC1626 switching regulator IC and was not designed
for any other purpose.
LTC1626 OPERATION
When the load is heavy, the LTC1626 is in continuous
mode, which operates as follows: the internal P-channel
MOSFET switch is turned on at the end of the off-time and
turned off when the inductor current has risen to the
current comparator threshold. During the off-time, the
catch diode D1 turns on. At the end of the off-time, the
P-channel MOSFET is again turned on, and the cycle
repeats.
SLEEP
S
4
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U
12
R
Q
+
S
–
V
TH2
–
V
TH1
T
+
OFF-TIME
CONTROL
C
5
T
However, when the load is relatively light, the LTC1626
automatically switches to Burst Mode operation. In this
mode, the output voltage is regulated by a hysteretic
comparator that, when tripped, shuts down the MOSFET
drive and control circuitry to conserve DC supply current.
From the time the comparator trips until the lower com-
parator threshold is reached, the load current is supplied
by the charge stored in the output capacitor. When the
output capacitor discharges to the lower threshold, the
main loop again briefly turns on at a low current level to
recharge the capacitor. This cycle repeats at progressively
slower rates as the output current is reduced.
As the input supply voltage drops near the output voltage,
the LTC1626 shifts smoothly from a high efficiency switch-
mode DC/DC regulator to a low dropout linear regulator
(i.e., 100% duty cycle). When the battery voltage rises
again, the LTC1626 smoothly shifts back to a high effi-
ciency DC/DC converter.
PWR V
1, 13
IN
PGND SW
14
C
–
I
6
TH
LB
3
OUT
V
2
IN
–
SENSE
V
FB
SGND
11
Figure 2. LTC1626 Block Diagram
+
SENSE
8
V
+
25mV to 150mV
V
OS
13k
G
+
REFERENCE
A3
–
–
SENSE
7
V
9
FB
SHDN
10
4 LB
IN
DC193 • BD