Datasheet

LT3692
17
3692fa
For more information www.linear.com/3692
applicaTions inForMaTion
Figure 7. BST Pin Considerations
V
IN
V
IN
V
X
> V
IN
+ 3V
BST
D2
GND
LT3692
SW
IND
V
OUT
V
BST
– V
SW
= V
X
V
BST(MAX)
= V
X
V
X(MIN)
= V
IN
+ 3V
V
OUT
< 3V
3692 F07
V
IN
V
IN
V
X
= LOWEST V
IN
OR V
OUT
> 3V
BST
D2
GND
LT3692
SW
IND
V
OUT
V
BST
– V
SW
= V
X
V
BST(MAX)
= V
IN
+ V
X
V
X(MIN)
= 3V
V
OUT
< 3V
C3
V
IN
V
IN
BST
D2
GND
(7d)(7c)
(7b)
LT3692
SW
IND
V
OUT
V
BST
– V
SW
= V
IN
V
BST(MAX)
= 2 •V
IN
V
OUT
< 3V
C3
V
IN
V
IN
BST
D2
GND
(7a)
LT3692
SW
IND
V
OUT
V
BST
– V
SW
= V
OUT
V
BST(MAX)
= V
IN
+ V
OUT
V
OUT
C3
the switch will not be able to saturate, dropping 1.5V to
2V in conduction.
The minimum input voltage of an LT3692 application is
limited by the minimum operating voltage (<3V) 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 LT3692 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
IN
is ramping very slowly.
Use a Schottky diode for the lowest start-up voltage.
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 LT3692 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3692 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
C
pin. Generally a capacitor and a resistor in series to
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.