LT3689 LINER [Linear Technology], LT3689 Datasheet - Page 13

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

APPLICATIONS INFORMATION
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose 1% resistors
according to:
For reference designators, refer to the Block Diagram.
Setting the Switching Frequency
The LT3689 uses a constant-frequency PWM architecture
that can be programmed to switch from 350kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. Table 1
shows the R
Table 1. Switching Frequency vs R
SWITCHING FREQUENCY (MHz)
R1= R2
0.8V
T
0.35
V
0.5
0.6
0.7
0.8
0.9
1.2
1.4
1.6
1.8
2.2
OUT
1
2
values for various switching frequencies.
1
T
R
T
31.6
24.9
20.5
16.9
14.7
12.7
9.53
6.04
4.87
4.02
3.16
48.7
7.5
(kΩ)
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
effi ciency, component size and maximum input voltage.
The advantage of high frequency operation is that smaller
inductor and capacitor values may be used. The disad-
vantages are lower effi ciency, and narrower input voltage
range at constant-frequency. The highest constant-switch-
ing frequency (f
calculated as follows:
where V
voltage, V
the internal switch drop (~0.5V at maximum load). If the
LT3689 is programmed to operate at a frequency higher
than f
enters pulse-skipping mode, where it skips switching
cycles to maintain regulation. At frequencies higher than
f
frequency. The LT3689 enters pulse-skipping mode at
frequencies higher than f
tion on the LT3689’s minimum on-time of 130ns. As the
switching frequency is increased above f
is required to switch for shorter periods of time to maintain
the same duty cycle. Delays associated with turning off
the power switch dictate the minimum on-time of the part.
When the required on-time decreases below the minimum
on-time of 130ns, the switch pulse width remains fi xed at
130ns (instead of becoming narrower to accommodate
the duty cycle requirement). The inductor current ramps
up to a value exceeding the load current and the output
ripple increases. The part then remains off until the output
voltage dips below the programmed value before it begins
switching again.
SW(MAX)
f
SW MAX
SW(MAX)
(
IN
, the LT3689 no longer operates with constant-
D
is the typical input voltage, V
)
is the catch diode drop (~0.5V) and V
=
for a given V
t
SW(MAX)
ON MIN
(
)
V
LT3689/LT3689-5
(
) for a given application can be
OUT
V
SW(MAX)
IN
IN
–
+
input voltage, the LT3689
V
V
SW
D
because of the limita-
+
V
D
OUT
SW(MAX)
)
is the output
, the part
13
SW
3689fa
is