LTC1624
U
W U U
APPLICATIONS INFORMATION
ITH/RUN pin below 0.8V relative to the LTC1624 ground
pin. With the LTC1624 ground pin referenced to –VOUT
Positive-to-negativeconvertershavehighripplecurrentin
both the input and output capacitors. For long capacitor
lifetime, the RMS value of this current must be less than
the high frequency ripple rating of the capacitor.
,
the nonimal range on the ITH/RUN pin is –VOUT (in
shutdown) to (–VOUT + 2.4V)(at Max IOUT). Referring to
Figure 15, M2, M3 and R3 provide a level shift from typical
TTL levels to the LTC1624 operating as positive-to-nega-
tive converter. MOSFET M3 supplies gate drive to M2
duringshutdown, whileM2pullstheITH/RUN pinvoltageto
–VOUT, shutting down the LTC1624.
ThefollowingformulagivesanapproximatevalueforRMS
ripple current. This formula assumes continuous mode
andlowcurrentripple.Smallinductorswillgivesomewhat
higher ripple current, especially in discontinuous mode.
For the exact formulas refer to Application Note 44, pages
28 to 30. The input and output capacitor ripple current
(occurring at VIN(MIN)) is:
Step-Down Converters: PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1624. These items are also illustrated graphically in
the layout diagram of Figure 9. Check the following in your
layout:
V
OUT
Capacitor I
= ff I
( )(
)
RMS
OUT
V
IN
ff = Fudge factor (1.2 to 2.0)
The output peak-to-peak ripple voltage is:
VOUT(P-P) = RESR (ID(MAX)
1. Are the signal and power grounds segregated? The
LTC1624 ground (Pin 4) must return to the (–) plate
of COUT.
)
2. Does the VFB (Pin 3) connect directly to the feedback
resistors? The resistive divider R1, R2 must be con-
nectedbetweenthe(+)plateofCOUT andsignalground.
The 100pF capacitor should be as close as possible to
the LTC1624.
The input capacitor can also see a very high surge current
when a battery is suddenly connected, and solid tantalum
capacitors can fail under this condition. Be sure to specify
surge tested capacitors.
Positive-to-Negative Converter: Duty Cycle
Limitations
3. Does the VIN lead connect to the input voltage at the
samepointasRSENSE andaretheSENSE– andVIN leads
routed together with minimum PC trace spacing? The
filter capacitor between VIN and SENSE– should be as
close as possible to the LTC1624.
The minimum on-time of 450ns sets a limit on how high
ofinput-to-outputratiocanbetoleratedwhilenotskipping
cycles. This only impacts designs when very low output
voltages ( VOUT < 2.5V) are needed. The maximum input
voltage is:
4. Does the (+) plate of CIN connect to RSENSE as closely
as possible? This capacitor provides the AC current to
the MOSFET(s). Also, does CIN connect as close as
possible to the VIN and ground pin of the LTC1624?
This capacitor also supplies the energy required to
recharge the bootstrap capacitor. Adequate input
decoupling is critical for proper operation.
VIN(MAX) < 10.1VOUT + 5V
For DC > 9%
VIN(MAX)<36V– VOUT Forabsolutemaximumratings
Positive-to-Negative Converter: Shutdown
Considerations
5. Keep the switch node SW away from sensitive small-
signal nodes. Ideally, M1, L1 and D1 should be con-
nected as closely as possible at the switch node.
Since the ground pin on the LTC1624 is referenced to
–VOUT, additional circuitry is needed to put the LTC1624
into shutdown. Shutdown is enabled by pulling the
20
Linear [ Linear ]
