LTC3851A_15 Datasheet, PDF (13/30 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851A
Applications Information
V_IN
V_IN
INTV_CC
BOOST
TG
LTC3851A
SW
BG
GND
SENSE⁺
SENSE^–
R1
C1* R2
INDUCTOR
L DCR
V_OUT
*PLACE C1 NEAR SENSE⁺, SENSE^– PINS
R1||R2
•
C1
=
L
DCR
R_SENSE(EQ)
=
DCR
R2
R1 + R2
3851A F02
Figure 2. Current Mode Control Using the Inductor DCR
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing sub-harmonic oscil-
lations at high duty cycles. It is accomplished intern­ ally
by adding a compensating ramp to the inductor current
signal. Normally, this results in a reduction of maximum
inductor peak cur­rent for duty cycles > 40%. However, the
LTC3851A uses a novel scheme that allows the maximum
inductor peak current to remain unaffected throughout all
duty cycles.
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
=
f
1
•L
V_OUT

^1–

V_OUT
V_IN



Accepting larger values of ∆I_L allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I_L = 0.3(I_MAX). The maximum
∆I_L occurs at the maximum input voltage.
The inductor value also has secondary effects. The tran­
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
≈10% of the current limit determined by R_SENSE. Lower
inductor values (higher ∆I_L) will cause this to occur at
lower load currents, which can cause a dip in efficiency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a
fixed 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 the
LTC3851A controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
3851afa
13