LT3992EFE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3992EFE' PDF : 36 Pages View PDF
Applications Information
D2
BST
VIN
VIN
SW
LT3992
VBST – VSW = VOUT
VBST(MAX) = VIN + VOUT
IND
VOUT
GND
C3
D2
VOUT
BST
VIN
VIN
SW
LT3992
VBST – VSW = VIN
VBST(MAX) = 2 • VIN
IND
VOUT
GND
VX = LOWEST VIN
OR VOUT > 3V
VIN
(7a)
D2
BST
VIN
SW
LT3992
VBST – VSW = VX
VBST(MAX) = VIN + VX
VX(MIN) = 3V
IND
VOUT
GND
(7c)
(7b)
D2
VX > VIN + 3V
C3
BST
VIN
VIN
SW
LT3992
VOUT < 3V
VBST – VSW = VX
VBST(MAX) = VX
VX(MIN) = VIN + 3V
IND
VOUT
GND
(7d)
Figure 7. BST Pin Considerations
LT3992
C3
VOUT < 3V
VOUT < 3V
3992 F07
The minimum input voltage of an LT3992 application is
limited by the minimum operating voltage (typically 2.9V)
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 LT3992 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 VIN 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 24V
output circuit in the Typical Applications section shows
the location of this resistor.
Frequency Compensation
The LT3992 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3992 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
VC 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.
For more information www.linear.com/LT3992
3992fa
17
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