lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 14

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3692A
The maximum input voltage is determined by the absolute
maximum ratings of the V
frequency and minimum duty cycle. The minimum duty
cycle is defined as:
Maximum input voltage as:
Note that the LT3692A will regulate if the input voltage is
taken above the calculated maximum voltage as long as
maximum ratings of the V
However operation in this region of input voltage will
exhibit pulse skipping behavior.
Example:
V
= 25°C, V
specification), V
14
OUT
DC
DC
DC
V
V
V
IN(MAX)
IN(MIN)
IN(MAX)
CLKOUT
MIN
= 3.3V, I
MAX
MIN
DCLKOSW1
SW1
SW2
Figure 5. Timing Diagram RT/SYNC = 28.0k,
t
P
= t
SW
= t
=
= 1μs, V
=
ON(MIN)
=
=
1+
ON(MIN)
= 0.1V, B = 50 (from boost characteristics
OUT
3.3 + 0.4
V
3.3 + 0.4
P
OUT
)
D
1
0.98
DC
50
0.14
= 1A, Frequency = 1MHz, Temperature
1
DIV
= 0.4V, t
MIN
• Frequency
+ V
= 98%
= 0.75V
• Frequency = 0.14
D
– 0.4 + 0.1= 3.48V
– 0.4 + 0.1= 26.1V
IN
– V
ON(MIN)
DCLKOSW2
IN
and BST pins are not violated.
P
D
P
and BST pins and by the
/2
P
+ V
= 140ns:
SW
P
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In cases where multiple input voltages are present, or the
V
channel 2, channel 1’s frequency can be divided by a factor
of 2, 4 or 8 from the programmed value by setting the DIV
pin resistor to the appropriate value. Dividing channel 1’s
frequency will increase the maximum input voltage by the
same ratio. Channel 1’s external components will have to
be chosen according to the resulting frequency.
Example:
V
= 25°C, V
specification), V
Inductor Selection and Maximum Output Current
A good first choice for the LT3692A inductor value is:
where f is frequency in MHz and L is in μH.
With this value the maximum load current will be ~3.5A,
independent of input voltage. The inductor’s RMS cur-
rent rating must be greater than your maximum load
current and its saturation current should be higher than
the maximum peak switch current, and will reduce the
output voltage ripple.
If the maximum load for a single channel is lower than
2.5A, then you can decrease the value of the inductor and
operate with higher ripple current, or you can adjust the
maximum switch current for the channel via the ILIM pin.
This allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency.
The peak inductor and switch current is:
To maintain output regulation, this peak current must be
less than the LT3692A’s switch current limit, ILIM. ILIM
IN
OUT
I
DC
L =
V
/V
SW(PK)
IN1(MAX)
OUT
= 3.3V, I
MIN1
V
OUT
ratio for channel 1 is significantly different than
f
SW
= t
= I
=
= 0.1V, B = 50 (from boost characteristics
OUT
L(PK)
ON(MIN1)
3.3 + 0.4
D
0.07
= 1A, Frequency = 1MHz, Temperature
= 0.4V, t
= I
OUT
• Frequency/2 = 0.07
– 0.4 + 0.1= >39V
ON(MIN)
+
ΔI
2
L
= 140ns. V
DIV
= 0.75V.
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