ltm4603hv Linear Technology Corporation, ltm4603hv Datasheet - Page 12

• Manufacturer Part Number: ltm4603hv
• Description: 6a, 28vin Dc/dc ?module With Pll, Output Tracking And Margining
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTM4603HV
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 2A
equals 1.2A of effective output ripple current. Refer to Ap-
plication Note 77 for a detailed explanation of output ripple
current reduction as a function of paralleled phases.
The output voltage ripple 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 voltage ripple can be calulated with
the known effective output ripple current. The equation:
ΔV
is frequency and m is the number of parallel phases. This
calclation process can be easily fulfi lled using the Linear
Technology μModule design tool.
12
OUT(P-P)
≈ (ΔI
L
/(8 • f • m • C
U
U
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
Figure 4. Normalized Output Ripple Current vs Duty Cycle, Dlr = V
0
0.1 0.15 0.2 0.25
OUT
) + ESR • ΔI
W
0.3
L
U
), where f
0.35
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
DUTY CYCLE (V
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4603HV has a current mode controller, which
inherently limits the cycle-by-cycle inductor current not
only in steady-state operation, but also in transient.
To further limit current in the event of an overload condition,
the LTM4603HV 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 minus any margin delta. This will control
O
/V
IN
)
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
O
4603HV F04
T/L
I
4603hvf