MIC2155_0911 Datasheet, PDF (18/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
The MIC2155/6 current limit circuit restricts the
maximum output current. If the load tries to draw
additional current the output voltage drops until it is no
longer within regulation limits. At this point (75% of
nominal output voltage) a hiccup current mode is
initiated to protect down stream loads from excessive
current during hard short circuits. This helps reduce the
overall power dissipation in the PWM converter
components during a fault.
MIC2155/2156
Accurate Method
For designs where ripple current is significant when
compared to I_OUT or for low duty cycle operation,
calculating the current setting resistor R_CS should take
into account that we are sensing the peak inductor
current and that there is a blanking delay of
approximately 100ns.
Figure 14. Overcurrent Sense Waveforms
The MIC2155/6 only senses current across the low side
MOSFET of Channel 1 since both channels operate in
parallel. This means the total output current limit is
approximately twice the calculated current limit.
Current Limit Setting
The current limit circuit responds to the peak inductor
current flowing through the low-side FET. The value of
R_CS can be estimated with the “simple” method or can be
more accurately calculated by taking the inductor ripple
current into account.
The Simple Method
Current limit can be quickly estimated with the following
equation:
R_CS = I_OUT/2 × R_DSON(MAX)/180µA.
Where: R_DSON is the maximum on-resistance of the low
side FET at the operating junction temperature
Figure 15. Overcurrent waveform
The equations to accurately calculate the current limit
resistor value are shown below:
I_PK
=
I_OUT
2
+
I_RIPPLE
2
I_RIPPLE
=
V_OUT × (1− D)
F_S × L
I_SET
= I_PK
−
V_OUT × T_DLY
L
R_CS
=
I_SET
× R_DSON(MAX)
I_CS(MIN)
D = Duty Cycle
F_S = Switching Frequency
L = Power inductor value
T_DLY = Current limit blanking time ~ 100ns
I_CS(min) = 180µA
Example:
Consider a 12V to 3.3V @ 30A converter with 1.5µH
power inductor and 90% efficiency at full load. Each
channel will supply 15A at a 500kHz (MIC2155)
switching frequency. The on-resistance of the low side
MOSFET is 6mΩ.
Using the simple method:
30A _{× 6mΩ}
R_CS =
2
180μA
= 500Ω
November 2009
18
M9999-111209-B