lm5085sdx National Semiconductor Corporation, lm5085sdx Datasheet - Page 11

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

Functional Description
OVERVIEW
The LM5085 is a PFET buck (step-down) DC-DC controller
using the constant on-time (COT) control principle. The input
operating voltage range of the LM5085 is 4.5V to 75V. The
use of a PFET in a buck regulator greatly simplifies the gate
drive requirements and allows for 100% duty cycle operation
to extend the regulation range when operating at low input
voltage. However, PFET transistors typically have higher on-
resistance and gate charge when compared to similarly rated
NFET transistors. Consideration of available PFETs, input
voltage range, gate drive capability of the LM5085, and ther-
mal resistances indicate an upper limit of 10A for the load
current for LM5085 applications. Constant on-time control is
implemented using an on-time one-shot that is triggered by
the feedback signal. During the off-time, when the PFET (Q1)
is off, the load current is supplied by the inductor and the out-
put capacitor. As the output voltage falls, the voltage at the
feedback comparator input (FB) falls below the regulation
threshold. When this occurs Q1 is turned on for the one-shot
period which is determined by the input voltage (V
R
increases the voltage at FB above the feedback comparator
threshold. For a buck regulator the basic relationship between
the on-time, off-time, input voltage and output voltage is:
where Fs is the switching frequency. Equation 1 is valid only
in continuous conduction mode (inductor current does not
reach zero). Since the LM5085 controls the on-time inversely
proportional to V
constant as V
that is equal to or less than the regulated output voltage Q1
is held on continuously (100% duty cycle) and V
proximately equal to V
The COT control scheme, with the feedback signal applied to
a comparator rather than an error amplifier, requires no loop
compensation, resulting in very fast load transient response.
The LM5085 is available in both an 8 pin MSOP package and
an 8 pin LLP package with an exposed pad to aid in heat dis-
sipation. An 8 pin MSOP package without an exposed pad is
also available.
REGULATION CONTROL CIRCUIT
The LM5085 buck DC-DC controller employs a control
scheme based on a comparator and a one-shot on-timer, with
the output voltage feedback compared to an internal refer-
ence voltage (1.25V). When the FB pin voltage falls below the
feedback reference, Q1 is switched on for a time period de-
termined by the input voltage and a programming resistor
(R
falls below the reference. Q1 is then switched on for another
on-time period. The output voltage is set by the feedback re-
sistors (R
output voltage is calculated as follows:
The feedback voltage supplied to the FB pin is applied to a
comparator rather than a linear amplifier. For proper opera-
tion sufficient ripple amplitude is necessary at the FB pin to
switch the comparator at regular intervals with minimum delay
and noise susceptibility. This ripple is normally obtained from
the output voltage ripple attenuated through the feedback re-
T
T
resistor. During the on-time the increasing inductor current
). Following the on-time Q1 remains off until the FB voltage
V
FB1
OUT
, R
IN
= 1.25V x (R
IN
is varied. If the input voltage falls to a level
FB2
, the switching frequency remains relatively
in the Block Diagram). The regulated
IN
.
FB2
+ R
FB1
)/ R
FB1
IN
OUT
) and the
is ap-
(1)
(2)
11
sistors. The output voltage ripple is a result of the inductor’s
ripple current passing through the output capacitor’s ESR, or
through a resistor in series with the output capacitor. Multiple
methods are available to ensure sufficient ripple is supplied
to the FB pin, and three different configurations are discussed
in the Applications Information section.
When in regulation, the LM5085 operates in continuous con-
duction mode at medium to heavy load currents and discon-
tinuous conduction mode at light load currents. In continuous
conduction mode the inductor’s current is always greater than
zero, and the operating frequency remains relatively constant
with load and line variations. The minimum load current for
continuous conduction mode is one-half the inductor’s ripple
current amplitude. In discontinuous conduction mode, where
the inductor’s current reaches zero during the off-time, the
operating frequency is lower than in continuous conduction
mode and varies with load current. Conversion efficiency is
maintained at light loads since the switching losses are re-
duced with the reduction in load and frequency.
If the voltage at the FB pin exceeds 1.6V due to a transient
overshoot or excessive ripple at V
age comparator immediately switches off Q1. The next on-
time period starts when the voltage at FB falls below the
feedback reference voltage.
ON-TIME TIMER
The on-time of the PFET gate drive output (PGATE pin) is
determined by the resistor (R
and is calculated from:
where R
The minimum on-time, which occurs at maximum V
not be set less than 150 ns (see Current Limiting section). The
buck regulator effective on-time, measured at the SW node
(junction of Q1, L1, and D1) is typically longer than that cal-
culated in Equation 3 due to the asymmetric delay of the
PFET. The on-time difference caused by the PFET switching
delay can be estimated as the difference of the turn-off and
turn-on delays listed in the PFET data sheet. Measuring the
difference between the on-time at the PGATE pin versus the
SW node in the actual application circuit is also recommend-
ed.
In continuous conduction mode, the inverse relationship of
t
as V
from:
where R
delay difference. To set a specific continuous conduction
mode switching frequency (F
from the following:
where R
12V, and t
ON
with V
IN
is varied. The operating frequency can be calculated
T
T
T
is in kΩ.
IN
is in kΩ. A simplified version of Equation 5 at V
is in kΩ, and t
D
= 100 ns, is:
results in a nearly constant switching frequency
D
is equal to 50 ns plus the PFET’s
S
), the R
T
) and the input voltage (V
OUT
T
the internal over-volt-
resistor is determined
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IN
, should
IN
IN
(3)
(4)
(5)
),
=