Datasheet

LTC3605A
9
3605afg
For more information www.linear.com/LTC3605A
PV
IN
/SV
IN
Voltage Differential
SV
IN
should be tied to PV
IN
with a low pass filter of 1Ω
to 10Ω and 0.1μF. For applications where PV
IN
and SV
IN
are tied to vastly different voltage potentials, though the
output voltage will remain in regulation, there will be an
offset in the internal on-time generator such that if SV
IN
is different than PV
IN
by more than 50% of the PV
IN
volt-
age, the resulting switching frequency will deviate from
the frequency programmed by the R
T
resistor and/or the
external clock synchronization frequency. In such applica-
tions, in order to return the switching frequency back to
the original desired frequency
,
R
T
resistor value can be
adjusted accordingly. However, the better alternative is
to tie the V
ON
pin to a voltage different than that of V
OUT
in order to negate the offset of the V
IN
differential. For
instance, if SV
IN
is 6V and PV
IN
is 12V, the resulting switch-
ing frequency may be slower than what’s programmed
by the R
T
resistor. Tying the V
ON
pin to a voltage half of
V
OUT
will negate the V
IN
offset and return the switching
frequency back to normal.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
V
OUT
= 0.6V • (1 + R2/R1)
The resistive divider allows the V
FB
pin to sense a fraction
of the output voltage as shown in Figure 1.
Programming Switching Frequency
Connecting a resistor from the RT pin to SGND programs
the switching frequency from 800kHz to 4MHz according
to the following formula:
Frequency (Hz) =
1.6e11
R
T
(W
)
The internal PLL has a synchronization range of ±30%
around its programmed frequency. Therefore, during
external clock synchronization be sure that the external
clock frequency is within this ±30% range of the RT pro
-
grammed frequency.
Output Voltage T
racking and Soft-Start
The LTC3605A allows the user to program its output voltage
ramp rate by means of the TRACK/SS pin. An internal 2µA
pulls up the TRACK/SS pin to INTV
CC
. Putting an external
capacitor on TRACK/SS enables soft starting the output
to prevent current surge on the input supply. For output
tracking applications, TRACK/SS can be externally driven
by another voltage source. From 0V to 0.6V, the TRACK/SS
voltage will override the internal 0.6V reference input to the
error amplifier, thus regulating the feedback voltage to that
of TRACK/SS pins. During this start-up time, the LTC3605A
will operate in discontinuous mode. When TRACK/SS is
above 0.6V, tracking is disabled and the feedback voltage
will regulate to the internal reference voltage.
Output Power Good
When the LTC3605A’s output voltage is within the ±10%
window of the regulation point, which is reflected back as
a V
FB
voltage in the range of 0.54V to 0.66V, the output
voltage is good and the PGOOD pin is pulled high with
an external resistor. Otherwise, an internal open-drain
pull-down device (12Ω) will pull the PGOOD pin low.
To prevent unwanted PGOOD glitches during transients
or dynamic V
OUT
changes, the LTC3605A’s PGOOD fall-
ing edge includes a blanking delay of approximately 52
switching cycles.
operaTion
Figure 1. Setting the Output Voltage
FB
R2
V
OUT
C
FF
R1
3605A F01
LTC3605A
SGND
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