LTC3828_15 Datasheet, PDF (16/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIONS INFORMATION
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 ΔI_L decreases with higher
inductance or frequency and increases with higher V_IN:
ΔI_L
=
1
(f)(L)
⎛
V_{OUT ⎝⎜} 1–
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ΔI_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 ΔI_L = 0.3(I_MAX). The maximum
ΔI_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
25% of the current limit determined by R_SENSE. Lower
inductor values (higher ΔI_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 decrease.
Inductor Selection
Usually, high inductance is preferred for small current
ripple and low core loss. Unfortunately, increased induc-
tance requires more turns of wire or small air gap of the
inductor, resulting in high copper loss or low saturation
current. Once the value of L is known, the actual inductor
must be selected. There are two popular types of core
material of commercial available inductors.
Ferrite core inductors usually have very low core loss and
are preferred at high switching frequencies, so design
goals can concentrate on copper loss and preventing
saturation. However, ferrite core saturates “hard”, which
means that inductance collapses abruptly when the peak
design current is exceeded. This results in an abrupt in-
crease in inductor ripple current and consequent output
voltage ripple. One advantage of the LTC3828 is its current
mode control that detects and limits cycle-by-cycle peak
inductor current. Therefore, accurate and fast protection
16
is achieved if the inductor is saturated in steady state or
during transient mode.
Powder iron inductors usually saturate “soft”, which
means the inductance drops in a linear fashion when the
current increases. However, the core loss of the powder
iron inductor is usually higher than the ferrite inductor. So
design with high switching frequency should pay attention
to the inductor core loss too.
Inductor manufacturers usually provide inductance, DCR,
(peak) saturation current and (DC) heating current ratings
in the inductor data sheet. A good supply design should
not exceed the saturation and heating current rating of
the inductor.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the LTC3828: 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_CC voltage.
This voltage is typically 5V during start-up. Consequently,
logic-level threshold MOSFETs must be used in most ap-
plications. The only exception is if low input voltage is ex-
pected (V_IN < 5V); then, sub-logic level threshold MOSFETs
(V_GS(TH) < 3V) should be used. Pay close attention to the
BV_DSS specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance, R_DS(ON), Miller capacitance, C_MILLER, input
voltage and maximum output current. Miller capacitance,
C_MILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C_MILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in V_DS. This result is
then multiplied by the ratio of the application applied V_DS
to the Gate charge curve specified V_DS. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle = ^VOUT
V_IN
3828fc