LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 22

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3690
Output Tracking and Sequencing
Output tracking and sequencing between voltage regulators
can be implemented using the LT3690’s SS and PG pins.
Figures 8 and 9 show several configurations for output
tracking and sequencing of the LT3690 and an additional
regulator. Independent soft-start for each channel is shown
in Figure 8a. The output ramp time for each output is set
by the soft-start capacitor as described in the soft-start
section.
Ratiometric tracking is achieved in Figure 8b by connecting
SS pins of two regulators together. In this configuration,
the SS pin current is set by the sum of the SS pin currents
of the two regulators, which must be taken into account
when calculating the output rise time.
By connecting a feedback network from OUT1 to the
SS pin with the same ratio that set the OUT2 voltage,
absolute tracking shown in Figure 9a is implemented. A
small OUT2 voltage offset will be present due to the SS
pin’s 2µA source current. This offset can be corrected by
slightly reducing the value of R2.
Figure 9b illustrates output sequencing. When V
within 10% of its regulated voltage, PG releases the SS
soft-start pin, allowing V
Synchronization
To select low-ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic output).
Synchronize the LT3690 oscillator to an external frequency
by connecting a square wave (with positive and negative
pulse width > 100ns) to the SYNC pin. The square wave
amplitude should have valleys that are below 0.4V and
peaks that are above 1V (up to 6V).
The LT3690 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will skip pulses to maintain regulation.
The LT3690 may be synchronized over a 170kHz to 1.5MHz
range. The R
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
350kHz and higher, choose R
22
T
resistor should be chosen to set the LT3690
OUT2
T
for 280kHz. To assure reliable
to soft-start.
OUT1
is
and safe operation, the LT3690 will synchronize when the
output voltage is above 90% of its regulated voltage. It is
therefore necessary to choose a large enough inductor value
to supply the required output current at the frequency set
by the R
also important to note that slope compensation is set by
the R
higher than the one set by R
significantly reduced, which may require a larger inductor
value to prevent sub-harmonic oscillation.
For duty cycles greater than 50% (V
minimum inductance is required to avoid sub-harmonic
oscillations:
where V
(0.12V at maximum load), ƒ
μH. For ƒ
programmed by R
Undervoltage Lockout
Figure 10 shows how to add undervoltage lockout (UVLO)
to the LT3690. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
L
MIN
T
value. When the synchronization frequency is much
T
= V
LS
resistor (see the Inductor Selection section). It is
SW
(
is the voltage drop of the low side switch
OUT
in the above calculation, use the frequency
V
C4
IN
Figure 10. Undervoltage Lockout
+ V
T
R3
R4
, not the synchronization frequency.
LS
)
UVLO
•
0.42MHz
V
IN
ƒ
1.25V
T
SW
SW
, the slope compensation is
2µA
is in MHz, and L
–
+
OUT
SLEEP
LT3690
/V
3690 F10
IN
> 0.5), a
MIN
is in
3690f