lt3688fe Linear Technology Corporation, lt3688fe Datasheet - Page 20

• Manufacturer Part Number: lt3688fe
• Description: Lt3688 - Dual 800ma Step-down Switching Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

LT3688
APPLICATIONS INFORMATION
will charge these pins to ~2V. The RUN/SS pins provide
a soft-start function that limits peak input current to the
circuit during start-up. This helps to avoid drawing more
current than the input source can supply or glitching the
input supply when the LT3688 is enabled. The RUN/SS pins
do not provide an accurate delay to start or an accurately
controlled ramp at the output voltage, both of which depend
on the output capacitance and the load current.
Synchronization
Synchronizing the LT3688 oscillator to an external fre-
quency can be done by connecting a square wave (with
positive and negative pulse width > 150ns) to the SYNC
pin. The square wave amplitude should have valleys that
are below 0.4V and peaks that are above 1.3V (up to 6V).
The LT3688 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, R
assure reliable and safe operation, the LT3688 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.
Shutdown and Undervoltage Lockout
Figure 8 shows how to add undervoltage lockout (UVLO)
to the LT3688. 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
20
T
, the slope compensation will be signifi cantly reduced,
T
resistor should be chosen to set the LT3688
T
should be chosen for 280kHz. To
T
resistor (see the
T
value. When
comparator will force the part into shutdown below the
minimum V
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 1.25V. Current hysteresis is added above the
EN threshold. This can be used to set voltage hysteresis
of the UVLO using the following:
Example: switching should not start until the input is above
4.40V, and is to stop if the input falls below 4V.
Keep the connection from the resistor to the EN/UVLO pin
short and make sure the interplane or surface capacitance
to switching nodes is minimized. If high resistor values
are used, the EN/UVLO pin should be bypassed with a
1nF capacitor to prevent coupling problems from the
switch node.
R3 =
R4 =
R3 =
R4 =
V
H
C1
= 4.40V, V
V
IN
R3
R4
V
V
4.40V – 4V
4.40V – 1.25V – 107k • 0.3μA
3.7μA
H
H
IN
3.7μA
– V
– 1.25V – R3 • 0.3μA
LT3688
EN/UVLO
of 3.5V. This feature can be used to prevent
Figure 8. Undervoltage Lockout
L
R3 • 1.25V
0.3μA
1.25V
107k • 1.25V
L
= 4V
= 107k
3.7μA
–
+
= 43.2k
V
C
RUN/SS
3688 F08
3688f