MIC2155 Datasheet, PDF (16/33 Pages) Micrel Semiconductor – 2-Phase, Single Output, PWM Synchronous Buck Control IC

Micrel, Inc.
V_DS
MIC2155/2156
to 500µA. The voltage divider resistors (R1 and R4 in
Figure 12) must be chosen to insure the output current
of the amplifier does not exceed the maximum of 500µA.
R1_MAX
≥
V_OUT − V_REF
500μA
where V_REF = 0.7V
A gain/phase plot of the remote sense amplifier in Figure
11 shows a typical 2MHz bandwidth. Phase lag is 45° at
1MHz.
Figure 10. dv/dt Induced Turn-On
The following steps can be taken to lower the gate drive
impedance, minimize the dv/dt induced current and
lower the FETs susceptibility to the induced glitch:
1) Chose a MOSFET with:
a) a high C_GS/C_GD ratio
b) a low internal gate resistance
2) Do not put a resistor between the LSD output
and the gate.
3) Insure both the gate drive and return etch are
short, low inductance connections.
4) Use a 4.5V V_GS rated MOSFET because its
higher gate threshold voltage is more immune to
glitches than a 2.5V or 3.3V rated FET.
5) Connect V_DD to V_IN or a 5V supply if V_IN is below
6V. The R_DSON of the internal driver will be lower
and a 4.5V rated MOSFET can be used.
Remote Sense
Remote sensing provides accurate output voltage
regulation by sensing at the load. Remote sensing
makes up for losses in the power distribution path. It
uses a unity gain differential amplifier to overcome
voltage drops in both the output and return (ground)
paths. The amplifier has common mode input range from
-0.3V to 3.6V. For proper remote sense operation, V_IN
must be greater than 6V. If V_IN is less than 6V, the V_DD
pin must be connected to V_IN or externally supplied with
5V.
The output of the remote sense amplifier can source up
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
May 2009
16
M9999-052709-A
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