lm3150mhx National Semiconductor Corporation, lm3150mhx Datasheet - Page 9

• Manufacturer Part Number: lm3150mhx
• Description: Simple Switcher Controller, 42v Synchronous Step-down
• Manufacturer: National Semiconductor Corporation

Where I
to-peak inductor ripple current.
If an overload condition occurs, the current through the low-
side switch will increase which will cause the current limit
comparator to trigger the logic to skip the next on-time cycle.
The IC will then try to recover by checking the valley current
during each off-time. If the valley current is greater than or
equal to I
allow the inductor current to further decay.
Throughout the whole process, regardless of the load current,
the on-time of the controller will stay constant and thereby the
positive ripple current slope will remain constant. During each
on-time the current ramps-up an amount equal to:
The valley current limit feature prevents current runaway con-
ditions due to propagation delays or inductor saturation since
the inductor current is forced to decay following any overload
conditions.
Current sensing is achieved by either a low value sense re-
sistor in series with the low-side FET or by utilizing
the R
is the preferred choice for a more simplified design and lower
costs. The R
coefficient and will increase in value as the FET’s temperature
increases. The LM3150 controller will maintain a more stable
current limit that is closer to the original value that was set by
the user, by positively adjusting the I
temperature increases. This does not provide an exact tem-
perature compensation but allows for a more tightly controlled
current limit when compared to traditional R
methods when the R
from room to maximum temperature and cause other com-
ponents to be over-designed. The temperature compensated
I
LM3150 in Celsius:
To calculate the R
substitute equation (10) into I
LIM-TH
I
LIM-TH
DS(ON)
is shown below where T
OUT
CL
(T
of the low-side FET. The R
is the average output current and ΔI
, then the IC will keep the low-side FET on and
J
DS(ON)
) = I
LIM-TH
LIM
value of a FET has a positive temperature
DS(ON)
value with temperature compensation,
x [1 + 3.3 x 10
value can change typically 140%
LIM-TH
J
is the die temperature of the
in equation (7).
FIGURE 1. Inductor Current - Current Limit Operation
-3
DS(ON)
LIM-TH
x (T
J
sensing method
value as the IC
- 27)]
DS(ON)
L
is the peak-
sensing
(10)
(8)
(9)
9
Short-Circuit Protection
The LM3150 will sense a short-circuit on the output by mon-
itoring the output voltage. When the feedback voltage has
fallen below 60% of the reference voltage, V
short-circuit mode of operation will start. During short-circuit
operation, the SS pin is discharged and the output voltage will
fall to 0V. The SS pin voltage, V
the rate determined by the SS capacitor and I
reaches 0.7V. During this re-ramp phase, if the short-circuit
fault is still present the output current will be equal to the set
current limit. Once the soft-start voltage reaches 0.7V the
output voltage is sensed again and if the V
V
repeats until the short-circuit fault is removed.
Soft-Start
The soft-start (SS) feature allows the regulator to gradually
reach a steady-state operating point, which reduces start-up
stresses and current surges. At turn-on, while VCC is below
the under-voltage threshold, the SS pin is internally grounded
and V
ramp V
the duration of start-up can be changed accordingly. The
start-up time can be calculated using the following equation:
Where t
soft-start pin source current, which is typically 7.7 µA (refer to
electrical table).
An internal switch grounds the SS pin if VCC is below the
under-voltage lockout threshold, if a thermal shutdown oc-
curs, or if the EN pin is grounded. By using an externally
controlled switch, the output voltage can be shut off by
grounding the SS pin.
During startup the LM3150 will operate in diode emulation
mode, where the low-side gate LG will turn off and remain off
when the inductor current falls to zero. Diode emulation mode
will allow start-up into a pre-biased output voltage. When soft-
start is greater than 0.7V, the LM3150 will remain in continu-
ous conduction mode. During diode emulation mode at
current limit the low-gate will remain off when the inductor
current is off.
ref
x 0.6 then the SS pin is discharged again and the cycle
OUT
FB
SS
from 0V to 0.6V. By changing the capacitor value,
is held at 0V. The SS capacitor is used to slowly
is measured in seconds, V
SS
, is then ramped back up at
ref
30053112
= 0.6V and I
ref
FB
x 0.6 (
is still below
SS
www.national.com
≈
until V
SS
0.36V),
is the
(11)
SS