LM1771
Application Information
THEORY OF APPLICATION
The LM1771 synchronous buck controller has a control
scheme that is referred to as adaptive on-time control. This
topology relies on a fixed switch on-time to regulate the
output voltage. This on-time is internally set by EEPROM
and is available with three different set-points to allow for
different frequency options. The LM1771 automatically ad-
justs the on-time during operation inversely with the input
voltage (VIN) to maintain a constant frequency. Therefore
the switching frequency during continuous conduction mode
is independent of the inductor and capacitor size unlike
hysteretic switchers.
At the beginning of the cycle the LM1771 turns on the high
side PFET for a fixed duration. This on-time is predetermined
(internally set by EEPROM and adjusted by V
IN
) and the
switch will not turn off until the timer has completed its
period. The PFET will then turn off for a minimum pre-
determined time period. This minimum T
OFF
of 150ns is
internally set and cannot be adjusted. This is to prevent false
triggering from occurring on the comparator due to noise
from the SW node transition. After the minimum T
OFF
period
has expired, the PFET will remain off until the comparator
trip-point has been reached. Upon passing this trip-point (set
at 0.8V at the feedback pin), the PFET will turn back on and
the process will repeat, thus regulating the output.
The NFET control is complementary to the PFET control with
the exception of a short dead-time to prevent shoot through
from occurring.
DEVICE OPERATION
Timing Opinion
Three versions of the LM1771 are available each with a
predetermined T
ON
set internally by EEPROM. This T
ON
setting will determine the switching frequency for the appli-
cation. Derivation and calculation of the switching frequen-
cy’s dependence on V
IN
and T
ON
can be seen in the follow-
ing section.
In a PWM buck switcher the following equations can be
manipulated to obtain the switching frequency. The first
equation shows the standard duty-cycle equation given by
the volts-seconds balance on the inductor with the following
equations defining standard relationships:
VOUT
0.8
1
1.2
1.5
1.8
2.5
3.3
500ns
485
606
727
909
1091
1515
2000
where,
α
= V
IN
x T
ON
To maintain a set frequency in an application,
α
is always
held constant by varying T
ON
inversely with V
IN
. The three
versions of the LM1771 are identified by the on times at a
V
IN
of 3.3V for consistency. For clarification see the table
below:
Product ID
LM1771S
LM1771T
LM1771U
T
ON
@
3.3V
0.5µs
1.0µs
2.0µs
α
(V µs)
1.65
3.3
6.6
The variation of T
ON
versus V
IN
can also be expressed
graphically. These graphs can be found in the typical curves
section of the datasheet.
With
α
being a constant regardless of the version of the
LM1771 used, equation [6] shows that the only dependent
variable remaining is V
OUT
. Since V
OUT
will be a constant in
any application, the frequency will also remain constant. The
switching frequency at which the application runs depends
upon the V
OUT
desired and the LM1771 version chosen. For
any V
OUT
, three frequency options (LM1771 versions) can
be selected. This can be seen in the table below. The rec-
ommended frequency range of operation is 100kHz to
1000kHz.
Timing Options
1000ns
242
303
364
455
545
758
1000
2000ns
121
152
182
227
273
379
500
Switching Frequency (kHz) of LM1771 based on output voltage and timing
option.
SHORT-CIRCUIT PROTECTION
The LM1771 has an internal short circuit comparator that
constantly monitors the feedback node (except during soft-
start). If the feedback voltage drops below 0.55V (equivalent
to the output voltage dropping below 68% of nominal), the
comparator will trip causing the part to latch off. The LM1771
will not resume switching until the input voltage is taken
below the UVLO threshold and then brought back into its
normal operating range, or the part is disabled then re-
enabled through the enable pin. The purpose of this function
is to prevent a severe short circuit from causing damage to
the application. Due to the fast transient response of the
LM1771 a severe short on the output causing the feedback
to drop would only occur if the load applied had an effective
resistance that approaches the PMOS R
DS(ON)
.
PRECISION ENABLE
The LM1771 features a precision enable circuit. If the volt-
age on the EN pin is 1.2V or greater, the part is enabled and
switching will occur. If the enable voltage falls below 1.2V,
T
ON
= D x T
P
Using these equations and solving for duty-cycle:
D = f
SW
x T
ON
Frequency can now be expressed as:
Or simply written as:
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