lm5085sdx National Semiconductor Corporation, lm5085sdx Datasheet - Page 14

• Manufacturer Part Number: lm5085sdx
• Description: 75v Constant On-time Pfet Buck Switching Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
must be reduced, or a PFET with a smaller Total Gate Charge
must be used.
SOFT-START
The internal soft-start feature of the LM5085 allows the reg-
ulator to gradually reach a steady state operating point at
power up, thereby reducing startup stresses and current
surges. Upon turn-on, when Vcc reaches its under-voltage
lockout threshold, the internal soft-start circuit ramps the feed-
back reference voltage from 0V to 1.25V, causing V
ramp up in a proportional manner. The soft-start ramp time is
typically 2.5 ms.
In addition to controlling the initial power up cycle, the soft-
start circuit also activates when the LM5085 is enabled by
releasing the RT pin, and when the circuit is shutdown and
restarted by the internal Thermal Shutdown circuit.
If the voltage at FB is below the regulation threshold value
due to an over-current condition or a short circuit at Vout, the
internal reference voltage provided by the soft-start circuit to
the regulation comparator is reduced along with FB. When the
over-current or short circuit condition is removed, V
turns to the regulated value at a rate determined by the soft-
start ramp. This feature helps prevent the output voltage from
over-shooting following an overload event.
THERMAL SHUTDOWN
The LM5085 should be operated such that the junction tem-
perature does not exceed 125°C. If the junction temperature
increases above that, an internal Thermal Shutdown circuit
activates at 170°C (typical) to disable the VCC regulator and
the gate driver, and discharge the soft-start capacitor. This
feature helps prevent catastrophic failures from accidental
device overheating. When the junction temperature falls be-
low 150°C (typical hysteresis = 20°C), the gate driver is
enabled, the soft-start circuit is released, and normal opera-
tion resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. Referring to the
Block Diagram, the circuit is to be configured for the following
specifications:
V
V
Maximum load current (I
Minimum load current (I
conduction mode)
Switching Frequency (F
Maximum allowable output ripple (V
Selected PFET: Vishay Si7465
R
ratio of these resistors is calculated from:
For this example, R
should be chosen from standard value resistors in the range
of 1 kΩ to 20 kΩ which satisfy the above ratio. For this ex-
ample, R
R
selected as its turn-on and turn-off delays affect the calculated
value of R
typical turn-off and turn-on delays is 57 ns. Using equation 5
at nominal input voltage, R
OUT
IN
FB1
T
, PFET: Before selecting the R
= 7V to 55V, 12V nominal
and R
= 5V
FB2
T
. For the Vishay Si7465 PFET, the difference of its
FB2
= 10 kΩ, and R
: These resistors set the output voltage. The
R
FB2
/R
FB2
FB1
/R
SW
OUT(max)
OUT(min)
FB1
= (V
) = 300 kHz
T
FB1
calculates to be:
= 3. Typically, R
OUT
= 3.4 kΩ.
) = 600 mA (for continuous
) = 5A
T
/1.25V) - 1
resistor, the PFET must be
OS
) = 5 mVp-p
FB1
and R
OUT
OUT
FB2
re-
to
14
A standard value 90.9 kΩ resistor is selected. Using equation
3 the minimum on-time at the PGATE pin, which occurs at
maximum input voltage (55V), is calculated to be 300 ns. This
minimum one-shot period is sufficiently longer than the mini-
mum recommended value of 150 ns. The minimum on-time
at the SW node is longer due to the delay added by the PFET
(57 ns). Therefore the minimum SW node on-time is 357 ns
at 55V. At the SW node the maximum on-time is calculated
to be 2.55 µs at 7V.
L1: The main parameter controlled by the inductor value is
the current ripple amplitude (I
load current for continuous conduction mode is used to de-
termine the maximum allowable ripple such that the inductor
current’s lower peak does not fall below 0 mA. Continuous
conduction mode operation at minimum load current is not a
requirement of the LM5085, but serves as a guideline for se-
lecting L1. For this example, the maximum ripple current is:
If an application’s minimum load current is zero, a good initial
estimate for the maximum ripple current (I
the maximum load current. The ripple calculated in equation
13 is then used in the following equation to calculate L1:
A standard value 15 µH inductor is selected. Using this in-
ductance value, the maximum ripple current amplitude, which
occurs at maximum input voltage, calculates to 1.19 Ap-p.
The peak current (I
ever, the current rating of the selected inductor must be based
on the maximum current limit value calculated below.
R
over designing the power stage components, the sense re-
sistor method is used for current limiting in this example. A
standard value 10 mΩ resistor is selected for R
in a 50 mV drop at maximum load current, and a maximum
0.25W power dissipation in the resistor. Since the LM5085
uses peak current detection, the minimum value for the cur-
rent limit threshold must be equal to the maximum load cur-
rent (5A) plus half the maximum ripple amplitude calculated
above:
At this current level the voltage across R
the current limit comparator offset of 9 mV (max) increases
the required current limit threshold to 6.5A. Using equation 7
SEN
, R
ADJ
FIGURE 3. Inductor Current Waveform
I
: To achieve good current limit accuracy and avoid
OR(max)
I
CL(min)
PK
= 2 x I
) at maximum load current is 5.6A. How-
= 5A + 1.19A/2 = 5.6A
OUT(min)
OR
). See Figure 3. The minimum
= 1.2 Amp
SEN
OR(max)
is 56 mV. Adding
SEN
) is 20% of
, resulting
30057732
(13)
(14)