LT3689 LINER [Linear Technology], LT3689 Datasheet - Page 22

• Manufacturer Part Number: LT3689
• Description: 700mA Step-Down Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

LT3689/LT3689-5
Synchronization
To select low ripple Burst Mode operation, tie the SYNC pin
below 0.3V (this can be ground or a logic output). Synchro-
nizing the LT3689 oscillator to an external frequency can
be done by connecting a square wave (with positive and
negative pulse width >80ns) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 1V (up to 6V).
The LT3689 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 LT3689 may be synchronized over a 350kHz to 2.5MHz
range. The R
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
350kHz and higher, the R
To assure reliable and safe operation, the LT3689 will only
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
Inductor Selection section). It is also important to note
that the slope compensation is set by the R
the sync frequency is much higher than the one set by
R
which may require a larger inductor value to prevent
subharmonic oscillation. The minimum inductor value
APPLICATIONS INFORMATION
22
T
, the slope compensation will be signifi cantly reduced,
T
resistor should be chosen to set the LT3689
T
should be chosen for 280kHz.
C1
V
IN
R3
R4
T
EN/UVLO
LT3689
resistor (see the
T
1.25V
value. When
Figure 9. Undervoltage Lockout
4μA
–
+
should be calculated using the RT programmed frequency
to avoid subharmonic oscillation.
Shutdown and Undervoltage Lockout
Figure 9 shows how to add undervoltage lockout (UVLO)
to the LT3689. 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. UVLO
prevents the regulator from operating at source voltages
where the problems might occur. An internal comparator
will force the part into shutdown below the minimum V
of 3.4V. This feature can be used to prevent excessive
discharge of battery-operated systems. If an adjustable
UVLO threshold is required, the EN/UVLO pin can be used.
The threshold voltage of the EN/UVLO pin comparator is
1.26V. Current hysteresis is added above the EN threshold.
This can be used to set voltage hysteresis of the UVLO
using the following:
R3 =
R4 =
V
V
H
4µA
H
V
R3
L
1.26V
1.26V
4k
V
SS
3689 F09
C
4µA
3689fa
IN