ML4903
DESIGN CONSIDERATIONS
(Continued)
hiccup mode are 1M
W
and 220nF, respectively. These
values will protect the external power components and
the power source from overheating during an overcurrent
condition. If it is necessary to change the ratio of on and
off times during overcurrent conditions, this can be done
by selecting a different value for C12. Larger values of
C12 will increase the delay between retry attempts (the
length of the “hiccup”), and smaller values will reduce
the delay.
HIGHER CURRENT LEVELS
Next generation processor chips will require currents of up
to 20A. Additionally, it is often desirable in larger
systems to distribute all power at one elevated voltage,
such as 12V, regulating it down to other voltages as
needed at the points of use. These applications are
readily met by the ML4903. For instance, the circuit
shown in Figure 1 will deliver an output current of 20A
with only three changes:
• As I
OUT
increases, the ripple current through the input
capacitor bank will also increase. Add at least one
820µF, 16V input capacitor in parallel with the three
shown (C1 - C3).
• Synchronous rectifier transistors Q3 and Q4 will see a
significantly greater RMS drain current at 20A output
than at 14A. Therefore, the use of lower R
DS(ON)
parts
such as Siliconix’ Si4420DY is required.
• The value of R1 may require adjustment, depending
upon factors such as the specific MOSFET type chosen
for Q3 and Q4, and the required operating ambient
temperature.
In dealing with circuits handling greater than 50W, it is
always important to pay attention to thermal issues.
When the circuit of Figure 1 is modified for >20A
applicatons, a key consideration is that it be provided
with adequate heatsinking. Ideally, the system should
provide 100 linear feet per minute (LFM) of airflow as
specified in Intel’s standards relating to VRMs. Micro
Linear does not recommend using the sense resistor
method of overcurrent protection at high output current
levels, as this does not provide the inherent thermal
foldback of I
OUT(MAX)
which is obtained by directly
sensing the V
DS(ON)
of the rectifier MOSFETs.
LAYOUT ISSUES
The two pins of the ML4903 which actually sense the
current limit voltage are I
SENSE
and GND. To facilitate
the required low-level sensing of the voltage between
these pins, there is no connection inside the ML4903
between GND and PWR GND. Because of this, there must
be an external connection between the ML4903 GND and
PWR GND pins. PWR GND must have a low impedance
connection to the ground plane used on the board, as high
instantaneous currents will flow in PWR GND when N
Given a –80mV trip point for the overcurrent comparator,
the value required for the sense resistor can be found by:
R
SENSE
=
|
-
107mV|
(125
I
OUT( MAX )
)
.
(4)
The power handling requirement for R
SENSE
is given by:
P
D
=
I
OUT( MAX)
2
V
´
1
-
V
´
R
!
OUT
IN
SENSE
"#
#$
(5)
For example, for a 14A output, R
SENSE
should be:
R
SENSE
=
|
−
80mV|
=
4.57m
Ω ≅
5.0m
Ω
125
×
14A
.
The power dissapated in R
SENSE
for a 12.0V input remains
at approximately 1W for all output voltages from 1.80V to
3.50V.
R
SENSE
must be a low inductance part, such as Dale/
Vishay’s type WSL-2512 series (WSL–2512–.005±1%).
Using a PCB trace as a current sense element is not
recommended due to the high temperature coefficient of
copper, and due to etching and plating tolerances which
can occur from board to board.
If a current sense resistor is not employed for overcurrent
protection, the voltage drop across (Q3||Q4)’s channel
during its conducting interval (the synchronous
rectification interval) is used to monitor the inductor
current. Ignoring the AC component of the current in the
buck inductor, the voltage across (Q3||Q4) will be:
V
SENSE
=
I
1
Q 3||Q 4
6
R
DS
0
ON
51
Q3||Q 4
6
(6)
R
DS(ON)
is typically specified at a MOSFET junction
temperature (T
j
) of 25ºC, but its value at other junction
temperatures can either be found graphically in the
MOSFET data sheet, or can be estimated by:
R
DS( ON)( T 2)
=
R
DS( ON)( 25ºC)
1007
T2
-
25º C
.
0
5
(7)
With a threshold of -80mV for the I
SENSE
comparator, the
current limit threshold is then:
I
LIMIT
=
-107
mV
R
DS( ON)( T 2)
(8)
For Pentium Pro and Pentium II applications, the
continuous current may be as high as 14A, so the current
limit threshold should be set for a minimum value of 16A
at the (Q3||Q4)’s highest anticipated T
J
. If necessary, the
voltage across the channel of (Q3||Q4) may be divided
using two moderately-valued resistors and presented after
that division to the ML4903.
The R and C values connected to the PROTECT pin for
setting the current limit delay and the off-time of the
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
