LTC3407EDD-2 Linear Technology, LTC3407EDD-2 Datasheet - Page 7

• Manufacturer Part Number: LTC3407EDD-2
• Description: IC REG DC/DC DUAL STEPDOWN 10DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3407EDD-2

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 5 V
• Current - Output: 1A
• Frequency - Switching: 1.5MHz
• Voltage - Input: 2.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-DFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATION
Low Current Operation
Two modes are available to control the operation of the
LTC3407-2 at low currents. Both modes automatically
switch from continuous operation to the selected mode
when the load current is low.
To optimize effi ciency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3407-2
automatically switches into Burst Mode operation, in which
the PMOS switch operates intermittently based on load
demand with a fi xed peak inductor current. By running
cycles periodically, the switching losses which are domi-
nated by the gate charge losses of the power MOSFETs
are minimized. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
A hysteretic voltage comparator trips when I
0.35V, shutting off the switch and reducing the power. The
output capacitor and the inductor supply the power to the
load until I
main control loop which starts another cycle.
For lower ripple noise at low currents, the pulse-skipping
mode can be used. In this mode, the LTC3407-2 continues
to switch at a constant frequency down to very low cur-
rents, where it will begin skipping pulses. The effi ciency in
APPLICATIONS INFORMATION
A general LTC3407-2 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, C
can be selected.
Inductor Selection
Although the inductor does not infl uence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ΔI
higher inductance and increases with higher V
Accepting larger values of ΔI
inductances, but results in higher output voltage ripple,
I
L
=
V
f
O
OUT
TH
• L
exceeds 0.65V, turning on the switch and the
• 1
V
V
OUT
IN
L
allows the use of low
L
decreases with
IN
TH
IN
and C
is below
or V
OUT
OUT
:
pulse-skipping mode can be improved slightly by connect-
ing the SW node to the MODE/SYNC input which reduces
the clock frequency by approximately 30%.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R
of the P-channel switch increases with decreasing input
supply voltage (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3407-2 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Low Supply Operation
To prevent unstable operation, the LTC3407-2 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 1.65V.
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
ΔI
largest ripple current ΔI
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes a dip in effi ciency in the upper
L
L
= 0.4 • I
f
O
V
OUT
• I
L
OUT(MAX)
• 1–
V
IN(MAX)
V
, where I
OUT
L
occurs at the maximum input
OUT(MAX)
LTC3407-2
is 800mA. The
DS(ON)
34072fc
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