Datasheet

LTM4603/LTM4603-1
13
4603fb
Figure 4 provides a ratio of peak-to-peak output ripple cur-
rent to the inductor current as a function of duty cycle and
the number of paralleled phases. Pick the corresponding
duty cycle and the number of phases to arrive at the correct
output ripple current ratio value. If a 2-phase operation
is chosen at a duty cycle of 21%, then 0.6 is the ratio.
This 0.6 ratio of output ripple current to inductor ripple
of 3A equals 8A of effective output ripple current. Refer
to Application Note 77 for a detailed explanation of output
ripple current reduction as a function of paralleled phases.
The output ripple voltage has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
Therefore, the output ripple voltage can be calculated with
the known effective output ripple current. The equation:
ΔV
OUT(P-P)
≈ (ΔI
L
/(8 • f • m • C
OUT
) + ESR • ΔI
L
), where
f is frequency and m is the number of parallel phases.
This calculation process can be easily accomplished by
LTpowerCAD™.
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4603 has a current mode controller which inher-
ently limits the cycle-by-cycle inductor current, not only in
steady-state operation but also in response to transients.
To further limit current in the event of an overload condi-
tion, the LTM4603 provides foldback current limiting. If the
output voltage falls by more than 50%, then the maximum
output current is progressively lowered to about one sixth
of its full current limit value.
Soft-Start and Tracking
The TRACK/SS pin provides a means to either soft-start
the regulator or track it to a different power supply. A
capacitor on this pin will program the ramp rate of the
output voltage. A 1.5µA current source will charge up the
external soft-start capacitor to 80% of the 0.6V internal
voltage reference plus or minus any margin delta. This will
Figure 4. Normalized Output Ripple Current vs Duty Cycle, Dlr = V
O
T/L
I
applications inForMation
DUTY CYCLE (V
O
/V
IN
)
0.1 0.15 0.2 0.25 0.350.3 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
4603 F04
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
PEAK-TO-PEAK OUTPUT RIPPLE CURRENT
DIr
RATIO =