LTC1649 Ver la hoja de datos (PDF) - Linear Technology
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LTC1649 Datasheet PDF : 16 Pages
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LTC1649
APPLICATIONS INFORMATION
The CPOUT pin can typically supply 50mA at 5V, adequate
to power the VCC and PVCC pins. This supply can also be
used to power external circuitry, but any additional current
drawn from CPOUT subtracts from the current available to
drive the external MOSFETs. Circuits with small external
MOSFETs can draw as much as 20mA or 30mA from
CPOUT without hindering performance. High output cur-
rent circuits with large or multiple external MOSFETs may
need every milliamp they can get from CPOUT, and external
loads should be minimized. The charge pump at PVCC1 is
more limited in its abilities, and should not be connected
to anything except PVCC1. In particular, do not connect a
bypass capacitor from PVCC1 to ground—it will steal
charge from the charge pump and actually degrade perfor-
mance.
Compensation and Transient Response
The LTC1649 voltage feedback loop is compensated at the
COMP pin; this is the output node of the internal gm error
amplifier. The loop can generally be compensated prop-
erly with an RC network from COMP to GND and an
additional small C from COMP to GND (Figure 5). Loop
stability is affected by inductor and output capacitor
values and by other factors. Optimum loop response can
be obtained by using a network analyzer to find the loop
poles and zeros; nearly as effective and a lot easier is to
empirically tweak the RC values until the transient recovery
looks right with an output load step.
Output transient response is set by three major factors: the
time constant of the inductor and the output capacitor, the
ESR of the output capacitor, and the loop compensation
components. The first two factors usually have much
more impact on overall transient recovery time than the
third; unless the loop compensation is way off, more
improvement can be had by optimizing the inductor and
the output capacitor than by fiddling with the loop com-
pensation components. In general, a smaller value induc-
tor will improve transient response at the expense of ripple
and inductor core saturation rating. Minimizing output
capacitor ESR will also help optimize output transient
response. See Input and Output Capacitors for more
information.
Soft Start and Current Limit
The LTC1649 includes a soft start circuit at the SS pin; this
circuit is used both for initial start-up and during current
limit operation. SS requires an external capacitor to GND
with the value determined by the required soft start time.
An internal 12µA current source is included to charge the
external capacitor. Soft start functions by clamping the
maximum voltage that the COMP pin can swing to, thereby
controlling the duty cycle (Figure 6). The LTC1649 will
begin to operate at low duty cycle as the SS pin rises to
about 2V below the VCC pin. As SS continues to rise, the
duty cycle will increase until the error amplifier takes over
and begins to regulate the output. When SS reaches 1V
below VCC the LTC1649 will be in full operation. An internal
switch shorts the SS pin to GND during shutdown.
LTC1649
COMP
RC
CC
C1
1659 F05
Figure 5. Compensation Pin Hook-Up
COMP
LTC1649
FB
VCC
SS
CSS
12µA
1649 F06
Figure 6. Soft Start Clamps COMP Pin
11
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