LT3689 Datasheet, PDF (22/32 Pages) Linear Technology – 700mA Step-Down Regulator with Power-On Reset and Watchdog Timer

LT3689/LT3689-5
APPLICATIONS INFORMATION
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_T resistor should be chosen to set the LT3689
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
350kHz and higher, the R_T should be chosen for 280kHz.
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_T resistor (see the
Inductor Selection section). It is also important to note
that the slope compensation is set by the R_T value. When
the sync frequency is much higher than the one set by
R_T, the slope compensation will be significantly reduced,
which may require a larger inductor value to prevent
subharmonic oscillation. The minimum inductor value
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_IN
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 = ^{V H VL} 4k
4µA
R4 =
V_H
1.26V
1.26V
4µA
R3
V_IN
LT3689
–
V_C
1.25V
R3
EN/UVLO
+
C1 R4
SS
4μA
3689 F09
Figure 9. Undervoltage Lockout
3689fa
22