ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 15

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher effi ciency at heavy loads.
Peak effi ciency is about the same with either method.
To maintain a good signal to noise ratio for the current
sense signal, use a minimum ΔV
For a DCR sensing application, the actual ripple voltage
will be determined by the equation:
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished inter-
nally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak cur-
rent for duty cycles > 40%. However, the LTC3850-2 uses
a patented scheme that counteracts this compensating
ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f
inductor’s peak-to-peak ripple current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
I
Δ
RIPPLE
V
SENSE
=
=
V
V
OUT
V
IN
IN
R C
−
1 1
⎛
⎝ ⎜
•
V
V
IN
OUT
f
OSC
– V
V
•L
OUT
IN
V
OUT
•
⎞
⎠ ⎟
OSC
f
OSC
SENSE
directly determine the
of 10mV to 15mV.
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fi xed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3850-2: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
This voltage is typically 5V during start-up (see EXTV
Connection). Consequently, logic-level threshold MOSFETs
must be used in most applications. The only exception is if
low input voltage is expected (V
threshold MOSFETs (V
attention to the BV
most of the logic level MOSFETs are limited to 30V or less.
L ≥
f
OSC
V
IN
– V
•I
RIPPLE
OUT
DSS
•
OUT(MAX)
specifi cation for the MOSFETs as well;
GS(TH)
V
V
OUT
IN
< 3V) should be used. Pay close
IN
. Note that the largest ripple
< 5V); then, sub-logic level
LTC3850-2
CC
voltage.
15
CC
38502f
Pin