lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 17

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

APPLICATIONS INFORMATION
The minimum input voltage of an LT3692A application is
limited by the minimum operating voltage (typically 2.8V)
and by the maximum duty cycle as outlined above. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3692A is turned on with its SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The Typical Performance Characteristics
section shows plots of the minimum load current to start
and to run as a function of input voltage for 3.3V outputs.
In many cases the discharged output capacitor will present
a load to the switcher which will allow it to start. The plots
show the worst-case situation where V
slowly. Use a Schottky diode for the lowest start-up voltage.
V
X
V
V
= LOWEST V
BST
BST(MAX)
OR V
V
V
V
BST
BST(MAX)
X(MIN)
– V
OUT
– V
SW
> 3V
= V
= 3V
V
SW
= V
V
IN
IN
= V
IN
IN
= V
OUT
+ V
IN
X
OUT
+ V
D2
X
V
V
IN
IN
LT3692A
LT3692A
(7a)
(7c)
GND
GND
BST
BST
V
V
IND
OUT
IND
OUT
SW
SW
IN
is ramping very
Figure 7. BST Pin Considerations
C3
C3
V
OUT
< 3V
D2
V
OUT
Outputs Greater Than 6V
For outputs greater than 6V, add a resistor of 1k to 2.5k
across the inductor to damp the discontinuous ringing of
the SW node, preventing unintended SW current. The 12V
output circuit in the Typical Applications section shows
the location of this resistor.
Frequency Compensation
The LT3692A uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3692A does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
ground determine loop gain. In addition, there is a lower
value capacitor in parallel. This capacitor is not part of
the loop compensation but is used to filter noise at the
switching frequency.
C
V
pin. Generally a capacitor and a resistor in series to
X
> V
V
V
BST
BST(MAX)
V
V
V
IN
BST
BST(MAX)
X(MIN)
– V
+ 3V
V
V
– V
IN
IN
SW
= V
SW
= V
= V
IN
= V
IN
+ 3V
X
D2
IN
X
D2
V
V
IN
IN
LT3692A
LT3692A
(7b)
(7d)
GND
GND
BST
BST
V
V
IND
OUT
IND
OUT
SW
SW
3692a F07
C3
LT3692A
V
V
OUT
OUT
< 3V
< 3V
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