LT1616ES6#TRM Linear Technology, LT1616ES6#TRM Datasheet - Page 7

• Manufacturer Part Number: LT1616ES6#TRM
• Description: IC SW REG STEP-DN 1.4MHZ SOT23-6
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1616ES6#TRM

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 3.3V, 5V
• Current - Output: 600mA
• Frequency - Switching: 1.4MHz
• Voltage - Input: 3.6 ~ 25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-6
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LT1616ES6#TRMTR

APPLICATIO S I FOR ATIO
Inductor Selection and Maximum Output Current
The duty cycle of the internal switch is:
where V
(D1) and V
Usually one is interested in DC at full load current, so you
can use V
maximum guaranteed duty cycle of 0.8. This will limit the
minimum input voltage for a particular output voltage.
When the switch is off, the inductor sees the output
voltage plus the catch diode drop. This gives the peak-to-
peak ripple current in the inductor:
where f is the switching frequency of the LT1616 and L is
the value of the inductor. The average inductor current is
equal to the output current, so the peak inductor current
will be the output current plus one half of the ripple
current:
To maintain output regulation, this peak current must be
less than the LT1616’s switch current limit I
least 630mA at low duty cycles, decreasing to 430mA at
80% duty cycle. The maximum output current is a function
of the chosen inductor value:
If the inductor value is chosen so that the ripple current is
small, then the available output current will be near the
switch current limit. A good approach is to choose the
inductor so that the peak-to-peak inductor ripple is equal
to one third of the switch current limit. This leads to:
and
These expressions depend on duty cycle and therefore on
input voltage. Pick a nominal input voltage to calculate L,
then check the maximum available output current at the
minimum and maximum input voltages.
DC = (V
I
I
L = 3(1 – DC)(V
I
LPK
OUT(MAX)
OUT(MAX)
I
L
= (1 – DC)(V
= I
D
OUT
is the forward voltage drop of the catch diode
D
OUT
SW
= V
= I
= (5/6)I
+ I
+ V
is the voltage drop of the internal switch.
LIM
SW
D
L
OUT
U
)/(V
– I
OUT
/2.
= 0.4V. Note that the LT1616 has a
LIM
+ V
IN
L
+ V
.
/2.
U
– V
D
D
)/(I
)/(L • f)
SW
LIM
+ V
• f)
W
D
)
LIM
U
. I
LIM
is at
If your application calls for output current less than
400mA, you may be able to relax the value of the inductor
and operate with higher ripple current. This may allow you
to pick a physically smaller inductor or one with a lower DC
resistance. Be aware that these equations assume con-
tinuous inductor current. If the inductor value is low or the
load current is light, then the inductor current may become
discontinuous. This occurs when I
of discontinuous mode operation, see Linear Technology
Application Note AN44. Also, high duty cycle operation
may require slightly higher inductor values to avoid sub-
harmonic oscillations. See AN19.
The maximum load current as a function of input voltage
is plotted in the Typical Performance Characteristics sec-
tion of this data sheet. Maximum load current for 3.3V and
5V outputs is shown for several values of L. At the highest
input voltages, the load current is limited by power dissi-
pation in the LT1616.
Choose an inductor that is intended for power applica-
tions. Table 1 lists several manufacturers and inductor
series. The saturation current of the inductor should be
above 0.5A. The RMS current rating should be equal to or
greater than output current. For indefinite operation into a
short circuit, the RMS current rating should be greater
than 0.7A. The DC resistance should be less than 0.5 in
order maintain circuit efficiency.
Capacitor Selection
A Buck regulator draws from its input a square wave of
current with peak-to-peak amplitude as high as the switch
current limit. The input capacitor (C1) must supply the AC
component of this current. An RMS current rating of
250mA is adequate for LT1616 circuits. The input capaci-
tor must bypass the LT1616 internal control circuitry and
any other circuitry that operates from the input source. A
1 F ceramic capacitor will satisfy both of these require-
ments. If the impedance of the input source is high (due to
long wires or filter components), additional bulk input
capacitance may be required. In high duty cycle applica-
tions (5V
capacitor to 2.2 F. It may be possible to achieve lower cost
by using an electrolytic capacitor (tantalum or aluminum)
IN
to 3.3V
OUT
, for example), increase the input
L
= 2I
OUT
LT1616
. For details
7