LTC3828 Datasheet, PDF (16/32 Pages) Linear Technology – Dual 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIO S I FOR ATIO
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 induc-
tance or frequency and increases with higher V_IN:
∆I_L
=
1
(f)(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 transi-
tion 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 ∆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 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
increase 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 atten-
tion 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 applications. The only exception is if low
input voltage is expected (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 volt-
age 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
3828f