MIC2155_0911 Datasheet, PDF (17/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
60
200
50
160
40
120
30
80
20 _Phase
40
10
0
0
-40
-10 ^Magnitude
-80
-20
-120
-30
-160
-40
1k
-200
10k 100k 1M 10M
FREQUENCY (Hz)
Figure 11. Remote Sense Amplifier Gain/Phase Plot
A typical remote sense configuration is shown in Figure
12. The output of the remote sense amplifier feeds a
voltage divider (R1, R4), which is connected to the
Channel 1 error amplifier. The divider and compensation
network for the remote sense are the same as for a local
sense configuration. The 10Ω resistors provide an
alternate feedback path if the remote sense connections
are removed or opened. The remote sense connections
should not be shorted or the output voltage will increase
close to V_IN. The OVP circuit in the controller will not
protect against this type of fault since the feedback pin
voltage will be 0V.
LOAD
+SENSE
–SENSE
Figure 12. Remote Sense
MIC2155/2156
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 V_OUT.
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 overcurrent (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.
November 2009
17
M9999-111209-B