6
LTC34 08
3408f
FU CTIO AL DIAGRA
U
U
W
+
–
+
–
+
–
+
–
+
–
BCMP
EA
I
BCMP
P-CHANNEL
FB
+
–
I
RCMP
+
–
I
COMP
6
5
RUN
OSC
SLOPE
COMP
OSC
FREQ
÷
2
1.2V
0.85V
SLEEP
REF
8
V
OUT
1
V
OUT
360k
180k
BURST
V
IN
S
R
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
Q
Q
5Ω
2
V
IN
7
SW
4
GND
3408 BD
3
9
V
IN
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The LTC3408 uses a constant frequency, current mode step-
down architecture. The main (P-channel MOSFET), syn-
chronous (N-channel MOSFET) and bypass (P-channel
MOSFET) switches are internal. During normal operation,
the internal main switch is turned on each cycle when the
oscillator sets the RS latch, and turned off when the cur-
rent comparator, I
COMP
, resets the RS latch. The peak in-
ductor current at which I
COMP
resets the RS latch, is con-
trolled by the output of error amplifier EA. When the load
current increases, it causes a slight decrease in the feed-
back voltage, FB, relative to the external reference, which
in turn, causes the EA amplifier’s output voltage to increase
until the average inductor current matches the new load cur-
rent. While the main switch is off, the synchronous switch
is turned on until the beginning of the next clock cycle.
The LTC3408 operates in forced continuous mode where
the inductor current is constantly cycled. In this mode, the
output voltage can respond quickly to the external refer-
ence voltage by sourcing or sinking current as needed.
Controlling the Output Voltage
The output voltage can be dynamically programmed from
0.3V to 3.5V using the REF input. Because the gain to V
OUT
from REF is internally set to 3, the corresponding input
range at REF is 0.1V to 1.167V. V
OUT
can be modulated
during operation by driving REF with an external DAC.
When REF exceeds 1.2V, a 0.08Ω internal bypass P-channel
MOSFET connects V
IN
to V
OUT
, dramatically reducing the
drop across the inductor and the main switch.
Short-Circuit Protection
A current sense comparator monitors the current across
the bypass P-channel MOSFET with a trip current of about
2.5A. When this current is exceeded during a V
OUT
short
to ground, the bypass P-channel MOSFET is immediately
turned off. The propagation delay of the current sensing
comparator, I
BCMP
, detecting an overcurrent condition to
turning off the bypass P-channel MOSFET is approxmately
100ns. Once the bypass P-channel MOSFET is off for about
10µs to 20µs, it is allowed to turn back on. The initial
current limit is then lowered to about 1.6A after the first
current limit trip. If the short to ground persists, the cur-
rent comparator will trip at the lower current limit, turning
V
IN
C
IN
†
10µF
CER
V
IN
2.7V
TO 5V
LTC3408
RUN
REFREF
4.7µH*
3403 F01
*
**
†
MURATA LQH32CN4R7M11
TAIYO YUDEN JMK212BJ475MG
TAIYO YUDEN JMK212BJ106MN
SW
V
OUT
GND
C
OUT
**
4.7µF
CER
V
OUT
3× V
REF
600mA
Figure 1. Typical Application
