LT3689EMSE-5#PBF Linear Technology, LT3689EMSE-5#PBF Datasheet - Page 16

• Manufacturer Part Number: LT3689EMSE-5#PBF
• Description: IC BUCK 5V 0.7A 16MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3689EMSE-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-MSOP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3689/LT3689-5
Choose an inductor using the previous inductor selection
equation to guarantee 700mA of output current. If using
a smaller inductor, check the DA current limit equation to
verify that the DA circuitry will not lower the switching
frequency.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
where f
is the value of the inductor. The peak inductor and switch
current is:
To maintain output regulation, this peak current must be
less than the LT3689’s switch current limit I
least 1.5A for at low duty cycles and decreases linearly
to 0.87A at DC = 85%. The maximum output current is a
function of the chosen inductor value.
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
preceding simple rule, determine the available inductors,
and choose one to meet cost or space goals. Next, use
these equations to check that the LT3689 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when I
than ∆I
Of course, such a simple design guide will not always
result in the optimum inductor for the application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If the
APPLICATIONS INFORMATION
16
I
I
∆I
SW(PK)
OUT(MAX)
L
SW
L
=
/2.
(1−DC)(V
is the switching frequency of the LT3689 and L
= I
L(PK)
= I
= 1.15A • (1− 0.28 • DC) −
L • f
LIM
= I
OUT
SW
−
OUT
∆I
2
+ V
L
+
D
∆I
)
2
L
∆I
2
LIM
L
OUT
. I
LIM
is less
is at
load is lower than 0.7A, decrease the value of the inductor
and operate with a higher ripple current. This allows the
use of a physically smaller inductor, or one with a lower
DCR resulting in higher efficiency. There are graphs in
the Typical Performance Characteristics section of this
data sheet that show the maximum load current as a
function of input voltage for several popular output volt-
ages. Low inductance may result in discontinuous mode
operation, which is okay but further reduces maximum
load current. For details of maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44. Finally, for duty cycles greater than
50% (V
to avoid subharmonic oscillations:
where L
is in MHz.
Input Capacitor
Bypass the input of the LT3689 circuit with a ceramic
capacitor of an X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. The minimum value of input capaci-
tance depends on the switching frequency. Use an input
capacitor of 1µF or more for switching frequencies be-
tween 1MHz to 2.2MHz, and 2.2µF or more for frequen-
cies lower than 1MHz. If the input power source has high
impedance, or there is significant inductance due to long
wires or cables, additional bulk capacitance may be nec-
essary. This can be provided with a lower performance
electrolytic capacitor. Step-down regulators draw current
from the input supply in pulses with very fast rise and
fall times. The input capacitor is required to reduce the
resulting voltage ripple at the LT3689 input and to force
this very high frequency switching current into a tight local
loop, minimizing EMI. A ceramic capacitor is capable of
this task, but only if it is placed close to the LT3689 and
the catch diode (see the PCB Layout section). A second
precaution regarding the ceramic input capacitor concerns
the maximum input voltage rating of the LT3689. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
L
MIN
OUT
MIN
=
1.4 V
/ V
is in µH, V
IN
(
> 0.5), a minimum inductance is required
OUT
f
SW
+ V
OUT
D
)
and V
D
are in volts, and f
3689fd
SW