LTC3611_15 Datasheet, PDF (13/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

Applications Information
V_OUT
R_VON1
30k
R_VON2
100k
R_C
C_C
C_VON
0.01µF
V_ON
LTC3611
I_TH
LTC3611
2.0
1.5
DROPOUT
REGION
1.0
V_OUT
INTV_CC
R_VON1
3k
(2a)
R_VON2
10k 10k
Q1
2N5087
C_VON
0.01µF
R_C
C_C
V_ON
LTC3611
I_TH
3611 F02
(2b)
Figure 2. Correcting Frequency Shift with Load Current Changes
Minimum Off-time and Dropout Operation
The minimum off-time, t_OFF(MIN), is the smallest amount
of time that the LTC3611 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_ON/(t_{ON +} t_OFF(MIN)). If the maximum duty cycle
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:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3611 develops a 0.6V reference voltage between
the feedback pin, V_FB, and the signal ground as shown in
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
V_OUT
=
0.6V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
0.5
0
0
0.25
0.50
0.75
1.0
DUTY CYCLE (V_OUT/V_IN)
3611 F03
Figure 3. Maximum Switching Frequency vs Duty Cycle
To improve the frequency response, a feedforward capaci-
tor C1 may also be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
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:
ΔI_L
=
⎛
⎜
⎝
V_OUT
fL
⎞
⎟
⎠
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
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 trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). The largest ripple current
occurs at the highest V_IN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
=
⎛
⎝⎜
f
V_OUT
ΔI_L(MAX)
⎞
⎠⎟
⎛
⎝⎜ ^1−
V_OUT
V_IN(MAX)
⎞
⎠⎟
3611fd
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