LTC3839 Datasheet, PDF (17/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

APPLICATIONS INFORMATION
LTC3839
LTC3839
V_OUTSENSE+ V_{OUTSENSE–}
R_FB2
R_FB1
C_IN +– _VIN
M_{T L}
POWER TRACE
PARASITICS
M_B
±V_DROP(PWR)
C_OUT1
I_LOAD
GROUND TRACE
PARASITICS
±V_DROP(GND)
C_{OUT2 ILOAD}
OTHER CURRENTS
FLOWING IN
SHARED GROUND
PLANE
3839 F02
Figure 2. Differential Output Sensing Used to Correct Line Loss Variations
in a High Power Distributed System with a Shared Ground Plane
to maintain low output ripple voltage. Conversely, raising
the operating frequency degrades efficiency but reduces
component size.
The switching frequency of the LTC3839 can be pro-
grammed from 200kHz to 2MHz by connecting a resistor
from the RT pin to signal ground. The value of this resistor
can be chosen according to the following formula:
R_T
[kΩ]
=
41550
f [kHz]
–
2.2
The overall controller system, including the clock PLL
and switching channels, has a synchronization range of
no less than ±30% around this programmed frequency.
Therefore, during external clock synchronization be sure
that the external clock frequency is within this ±30% range
of the RT programmed frequency. It is advisable that the
RT programmed frequency be equal the external clock for
maximum synchronization margin. Refer to the “Phase
and Frequency Synchronization” section for more details.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. A higher frequency
generally results in lower efficiency because of MOSFET
gate charge losses. In addition to this basic trade-off, the
effect of inductor value on ripple current and low current
operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I_L decreases with higher
inductance or frequency and increases with higher V_IN:
ΔI_L
=
⎛
⎝⎜
V_OUT
f •L
⎞
⎠⎟
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ∆I_L allows the use of low induc-
tances, but results in higher output voltage ripple, higher
ESR losses in the output capacitor, and greater core losses.
A reasonable starting point for setting ripple current is ∆I_L
= 0.4 • I_MAX. The maximum ∆I_L occurs at the maximum
input voltage. To guarantee that ripple current does not
exceed a specified maximum, the inductance should be
chosen according to:
L
=
⎛
⎜
⎝
f
•
V_OUT
ΔI_{L(MAX )}
⎞
⎟
⎠
⎛
⎜
⎝
1–
V_OUT
V_{IN(MAX )}
⎞
⎟
⎠
3839fa
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