Datasheet

LTC3883/LTC3883-1
48
3883fa
For more information www.linear.com/LTC3883
APPLICATIONS INFORMATION
If the SYNC signal is not clocking in the application, the
PLL will run at the lowest free running frequency of the
VCO. This will be well below the intended PWM frequency
of the application and may cause undesirable operation
of the converter.
If the PWM signal appears to be running at too high a
frequency, monitor the SYNC pin. Extra transitions on
the falling edge will result in the PLL trying to lock on to
noise versus the intended signal. Review routing of digital
control signals and minimize crosstalk to the SYNC signal
to avoid this problem. Multiple LTC3883s are required to
share the SYNC pin in PolyPhase configurations, for other
configurations it is optional. If the SYNC pin is shared be
-
tween LTC3883s, only one LTC3883 can be programmed
with
a frequency output. All the other LTC3883s must be
programmed to external clock.
MINIMUM ON-TIME CONSIDERATIONS
Minimum on-time, t
ON(MIN)
, is the smallest time duration
that the LTC3883 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn off the top MOSFET. Low duty
cycle applications may approach
this minimum
on-time
limit and care should be taken to ensure that:
t
ON(MIN)
<
V
OUT
V
IN
• f
OSC
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The minimum on-time for the LTC3883 is approximately
90ns, with reasonably good PCB layout, minimum 30%
inductor current ripple and at least 10mV – 15mV ripple
on the current sense signal. The minimum on-time can be
affected by PCB switching noise in the voltage and cur
-
rent loop. As the peak current sense voltage decreases,
the
minimum on-time gradually increases to 130ns. This
is of particular concern in forced continuous applications
with low ripple current at light loads. If the duty cycle
drops below the minimum on-time limit in this situation,
a significant amount of cycle skipping can occur with cor
-
respondingly larger current and voltage ripple.
I
NPUT CURRENT SENSE AMPLIFIER
The LTC3883 input current sense amplifier can sense the
supply current into the V
IN
pin using an internal sense
resistor as well as the power stage current using an
external sense resistor. High frequency noise caused by
the discontinuous input current can cause input current
measurement errors. The
noise will be the greatest in
high
current applications and at large step-down ratios.
Care must be taken to mitigate the noise seen at the input
current sense amplifier inputs and supply. This can be
accomplished by careful layout as well as filtering at the
V
IN
, V
IN_SNS
and I
INSNS
pins. The V
IN
pin should be filtered
with a resistor and a ceramic capacitor located as close
to the V
IN
pin as possible. The supply side of the V
IN
pin
filter should be Kelvin connected to the supply side of the
R
IINSNS
resistor. A 3Ω resistor should be sufficient for
most applications. The resistor will cause an IR voltage
drop from the supply to the V
IN
pin due to the current
flowing into the V
IN
pin. To compensate for this voltage
drop, the MFR_RVIN command value should be set to
the nominal resistor value. The LTC3883 will multiply
the MFR_READ_ICHIP measurement value by the user
defined MFR_RVIN value and add this voltage to the
measured voltage at the V
IN
pin. Therefore READ_VIN =
V
VIN_PIN
+ (MFR_READ_ICHIP • MFR_RVIN), so that this
command will return the value of the voltage at the supply
side
of the V
IN
pin filter. If no V
IN
filter element is used,
set MFR_RVIN = 0.
10µF
R
IINSNS
100Ω
M1
M2
1µF
10µF10nF
10nF
TG
BG
SW
I
IN_SNS
V
IN_SNS
V
IN
LTC3883
V
IN
3Ω
3883 F25
100Ω
Figure 25. Low Noise Input Current Sense Circuit