MC33567
http://onsemi.com
5
Sense:
If the load is located away from the regulator, the voltage
drop on the connecting cable or trace can become
significant. The MC33567 provides tight voltage load
regulation with varying load currents using it’s SENSE
feature. As shown in Figure 5, the MC33567 senses the
voltage at the load and provides feedback to the regulator.
The regulator voltage is then adjusted to compensate for the
load changes. It is recommended that the SENSE connection
be placed as close as possible to the load. Also, use a separate
trace to connect the source of the Nchannel MOSFET to the
load to avoid interference or coupling with the SENSE
signal. The use of the SENSE feature is required for correct
device operation. If the SENSE pin is not connected to the
load, the output will go into Hiccup mode.
The current into the SENSE pin is given by the following
equation:
ISENSE
100 A
Vout
1.8 k
Figure 5. Voltage Regulation Using Sense Feature
SENSE
Vin
RL
DRV
Feedback
to
Regulator
1.8 k
+
100 A
ISENSE
NChannel MOSFET Selection:
The
MC33567
ON Semiconductor’s MTD3055VL Nchannel MOSFET.
Other MOSFETs can be used with the MC33567 as long as
power and stability requirements are met.
was
characterized
using
Power:
A MOSFET with a low drainsource on resistance
(RDS(on)) will insure the output voltage is not drastically
reduced due to excessive voltage drop across the MOSFET.
The required RDS(on) can be calculated using the equation
below:
RDS(on)
0.5
Vin
Vout
ILOAD
where:
Vin
Vout = Regulator Output Voltage
(1.2 V, 1.515 V, 1.818 V, 2.3 V, or 2.525 V)
ILOAD Load Current
A safety margin of 0.5 was added to account for RDS(on)
variations over the operating temperature range.
= Input Voltage, typically 3.3 V
Stability:
After evaluating the regulator, driver and load system
using control theory it is demonstrated that the output
capacitor, external driver gain and error amplifier gain
bandwidth play an important role on the system stability. To
insure system stability the following set of design guidelines
should be followed:
Ci
Cgs
Cgd
f
1
Ci· Ro
p
1
1
1
f
1
1
20 · 1
a
(3 ·
p)
·
1
gm
Rs
(3 ·
p)
a
·
1
gm
Co· Rs
5 ·
1
a
1
p
where:
f = Driver pole frequency
Ci = Input and reverse transfer capacitance when device
is off
Ro = Regulator output resistance (50 for the MC33567)
p= Secondary pole for open loop
a = Error amplifier gain bandwidth
1= Error amp second pole (set 1 = a, if not specified)
Rs = Output capacitor ESR
gm= Maximum driver transconductance gain
Co = Output capacitance
T
= Overall loop response time