LT3689EUD-5#PBF Linear Technology, LT3689EUD-5#PBF Datasheet - Page 20

• Manufacturer Part Number: LT3689EUD-5#PBF
• Description: IC BUCK 5V 0.7A 16QFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3689EUD-5#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 700mA
• Frequency - Switching: 350kHz ~ 2.2MHz
• Voltage - Input: 3.6 ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-WQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3689/LT3689-5
BST and OUT Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost voltage
that is higher than the input voltage. In most cases, a 0.1µF
capacitor will work well. Figure 6 shows three ways to ar-
range the boost circuit. The BST pin must be more than
2.3V above the SW pin for best efficiency. For outputs of
3V and above, the standard circuit (Figure 6a) is best. For
outputs between 2.8V and 3V, use a 0.47µF boost capaci-
tor. A 2.5V output presents a special case because it is
marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BST pin
operation with 2.5V outputs, use a good external Schottky
diode (such as ON Semiconductor’s MBR0540), and a
0.47µF boost capacitor (see Figure 6b). For lower output
voltages, the boost diode can be tied to the input (Figure 6c),
or to another supply greater than 2.8V. The circuit in
Figure 6a is more efficient because the BST pin current
and OUT pin quiescent current comes from a lower
APPLICATIONS INFORMATION
20
2.2µF
V
IN
V
IN
(6a) For V
LT3689
GND
OUT
BST
SW
OUT
Figure 6. Three Circuits for Generating the Boost Voltage
2.2µF
V
> 2.8V
IN
C3
(6c) For V
V
IN
LT3689
OUT
GND
OUT
V
OUT
< 2.5V; V
BST
SW
2.2µF
V
voltage source. Ensure that the maximum voltage ratings
of the BST and OUT pins are not exceeded.
The minimum operating voltage of an LT3689 application
is limited by the minimum input voltage (3.7V) and by the
maximum duty cycle, as outlined in the Minimum Operating
Voltage Range section. For proper start-up, the minimum
input voltage is also limited by the boost circuit. If the input
voltage is ramped slowly, or the LT3689 is turned on with
its EN/UVLO pin when the output is already in regulation,
then the boost capacitor may not be fully charged. Because
the boost capacitor is charged with the energy stored in
the inductor, the circuit will rely on some minimum load
current to get the boost circuit running properly. This
minimum load will depend on input and output voltages,
and on the arrangement of the boost circuit. The minimum
load generally goes to zero once the circuit has started.
For lower start-up voltage, the boost diode can be tied to
V
the absolute maximum rating of the BST pin.
IN
IN
IN(MAX)
; however, this restricts the input range to one-half of
C3
(6b) For 2.5V < V
V
= 30V
IN
3689 F06
LT3689
OUT
GND
V
OUT
BST
SW
OUT
C3
< 2.8V
D2
V
OUT
3689fd