MIC2155 Datasheet, PDF (17/33 Pages) Micrel Semiconductor – 2-Phase, Single Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Setting the Output Voltage
Regardless of whether the remote sensing or local
output voltage sensing is used, the output voltage is set
with voltage divider resistors R1 and R4 (Figure 12). The
equation below is used to calculate Vout.
V_OUT
=
V_REF
×
⎢⎣⎡¹⁺
R1 ⎤
R4 ^⎥⎦
Where V_REF=0.7V
Current Limit and Overcurrent Protection
The MIC2155/6 uses the synchronous (low-side)
MOSFETs R_DSON to sense an over current condition. The
low-side MOSFET is used because it displays lower
parasitic oscillations after switching then the upper
MOSFET. Additionally, it improves the accuracy and
reduces false tripping at lower voltage outputs and
narrow duty cycles since the off-time increases as duty
cycle decreases.
MIC2155/6
V_IN
Current Limit
200µA
HSD Q1
CS1 ^RCS
LSD Q2
–I_L × R_DSON
I_L
I_L
L1
C_O
Figure 13. Overcurrent Circuit
Inductor current flows from the lower MOSFET source to
the drain during the off-time. The drain voltage becomes
negative with respect to ground as the inductor current
continues to flow from Source to Drain. This negative
voltage is proportional to instantaneous inductor current
times the MOSFET R_DSON. The voltage across the low-
side FET becomes even more negative as the output
current increases. The overcurrent circuit operates by
passing a known fixed current source (200µA) through a
resistor R_CS. This sets up an offset voltage (I_CS × R_CS)
that is compared to the VDS of the low-side FET. When
I_SD (Source to Drain current) × I_L is equal to this voltage,
the MIC2155’s over current trigger is set, which disables
the next high side gate drive pulse. After missing the
high side pulse, the over current (OC) trigger is reset. If
on the next low side drive cycle, the current is still too
high i.e. V_CS is ≤ 0V, another high side pulse is missed
and so on. This effectively reduces the overall energy
transferred to the output and V_OUT starts to fall.
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
MIC2155/2156
current during hard short circuits. This helps reduce the
overall power dissipation in the PWM converter
components during a fault.
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
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.
May 2009
17
M9999-052709-A
[phone]