LTC3609EWKG#PBF Linear Technology, LTC3609EWKG#PBF Datasheet - Page 13

• Manufacturer Part Number: LTC3609EWKG#PBF
• Description: IC DC/DC CONV STP-DWN 6A 52-QFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3609EWKG#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 36 V
• Current - Output: 6A
• Voltage - Input: 4 ~ 32 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 52-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -

applicaTions inForMaTion
Minimum Off-time and Dropout Operation
The minimum off-time, t
of time that the LTC3609 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of t
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3609 develops a 0.6V reference voltage between
the feedback pin, V
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
Figure 2. Correcting Frequency Shift with Load Current Changes
V
V
IN MIN
OUT
(
= 0.6V 1+
ON
INTV
)
V
V
/(t
=
OUT
OUT
CC
V
ON
OUT



2N5087
R
R
R
+ t
10k
30k
VON2
100k
VON1
VON1
3k
FB
t
OFF(MIN)
Q1
R2
R1
ON
, and the signal ground as shown in



R
10k
+
VON2
C
OFF(MIN)
R
C
t
t
C
ON
OFF MIN
(2a)
(2b)
). If the maximum duty cycle
(
C
0.01µF
C
0.01µF
C
, is the smallest amount
VON
VON
C
R
)
C
V
I
V
I
TH
TH
LTC3609
LTC3609
ON
ON
3609 F02
To improve the frequency response, a feed-forward capaci-
tor, C1, may also be used. Great care should be taken to
route the V
inductor or the SW line.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
occurs at the highest V
does not exceed a specified maximum, the inductance
should be chosen according to:
L =
ΔI
Figure 3. Maximum Switching Frequency vs Duty Cycle
L




=
f ΔI




V
V
L(MAX)
FB
f L
OUT
OUT
2.0
1.5
1.0
0.5
0
line away from noise sources, such as the
0







1−

 1−






V
OUT(MAX)
0.25
V
OUT
IN
DUTY CYCLE (V
IN
V
. To guarantee that ripple current
IN(MAX)
V



OUT
0.50
. The largest ripple current




OUT
DROPOUT
REGION
/V
IN
0.75
)
LTC3609
3609 F03
1.0

3609fb