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Current Limitations
Peak current for a buck converter is limited by the maximum switch current rating. This
current rating is 4.5A up to 50% duty cycle (DC), decreasing to 3.7A at 80% duty cycle
for the MSK5059.
Figure 5
This is shown graphically in Figure 5, and can be calculated using the formula below:
I
P
= 4.5A for DC < 50%
I
P
= 3.21 + 5.95(DC) – 6.75(DC)
2
for 50% < DC < 90%
DC = Duty cycle = V
OUT
/V
IN
Maximum output current is then reduced by one-half peak-to-peak inductor current.
I
MAX
= I
P
– (V
OUT
)(V
IN
– V
OUT
)/2(L)(f)(V
IN
)
Example: with VOUT = 5V, VIN = 8V; DC = 5/8 = 0.625, L = 3.3µH
I
P
= 3.21 + 5.95(0.625) – 6.75(0.625)2 = 4.3A
I
MAX
= 4.3 – (5)(8-5)/2(3.3µH)(500kHz)(8) = 3.73A (Figure 6)
Figure 6
Current rating decreases with duty cycle because the RH1959 has internal slope
compensation to prevent current mode subharmonic switching. The RH1959 has
nonlinear slope compensation, which gives better compensation with less reduction in
current limit.