Datasheet

Design Procedure (Continued)
within 30ns. An input capacitance of less than 1000pF is
recommended. Several suitable MOSFETs are shown in
Table 1.
EXTERNAL SCHOTTKY DIODE (Syncronous)
A Schottky diode is recommended to prevent the intrinsic
body diode of the low-side MOSFET from conducting during
the deadtime in PWM operation. If the body diode turns on,
there is extra power dissipation in the body diode because of
the reverse-recovery current and higher forward voltage
drop. In addition, the high-side MOSFET has more switching
loss because the diode reverse-recovery current adds to the
high-side MOSFET turn-on current. These losses degrade
the efficiency by 1-2%. The improved efficiency and noise
immunity with the Schottky diode become more obvious with
increasing input voltage and load current.
It is important to place the diode very close to the switch pin
of the LM2655. Extra parasitic impedance due to the trace
between the switch pin and the cathode of the diode will
cause the current limit to decrease. The breakdown voltage
rating of the diode is preferred to be 25% higher than the
maximum input voltage. Since it is on for a short period of
time, the diode’s average current rating need only be 30% of
the maximum output current.
EXTERNAL SCHOTTKY DIODE (Asyncronous)
In asyncronous mode, the output current commutates
throught the schottky diode when the high-side MOSFET is
turned off. Using a schottky diode with low forward voltage
drop will minimize the effeciency loss in the diode. However,
to achieve the greatest efficiency, the LM2655 should be
operated in syncronous mode using a low-side MOSFET.
Since the Schottky diode conducts for the entire second half
of the duty cycle in asyncronous mode, it should be rated
higher than the full load current.
BOOST CAPACITOR
The boost capacitor provides the extra votage needed to
turn the high-side, n-channel MOSFET on. A 0.1 µF ceramic
capacitor is recommended for the boost capacitor. The typi-
cal voltage across the boost capacitor is 6.7V.
SOFT-START CAPACITOR
A soft-start capacitor is used to provide the soft-start feature.
When the input voltage is first applied, or when the SD(SS)
pin is allowed to go high, the soft-start capacitor is charged
by a current source (approximately 2 µA). When the SD(SS)
pin voltage reaches 0.6V (shutdown threshold), the internal
regulator circuitry starts to operate. The current charging the
soft-start capacitor increases from 2 µA to approximately
10 µA. With the SD(SS) pin voltage between 0.6V and 1.3V,
the level of the current limit is zero, which means the output
voltage is still zero. When the SD(SS) pin voltage increases
beyond 1.3V, the current limit starts to increase. The switch
duty cycle, which is controlled by the level of the current limit,
starts with narrow pulses and gradually gets wider. At the
same time, the output voltage of the converter increases
towards the nominal value, which brings down the output
voltage of the error amplifier. When the output of the error
amplifier is less than the current limit voltage, it takes over
the control of the duty cycle. The converter enters the normal
current-mode PWM operation. The SD(SS) pin voltage is
eventually charged up to about 2V.
The soft-start time can be estimated as:
T
SS
=C
SS
* 0.6V/2 µA + C
SS
* (2V−0.6V)/10 µA
During start-up, the internal circuit is monitoring the soft-start
voltage. When the softstart voltage reaches 2V, the under-
voltage and overvoltage protections are enabled.
If the output voltage doesn’t rise above 80% of the normal
value before the soft-start reaches 2V, undervoltage protec-
tion shut down the device. You can avoid this by either
increasing the value of the soft-start capacitor, or using a
LDELAY capacitor.
LDELAY CAPACITOR
The LDELAY capacitor (CDELAY) provides a means to con-
trol undervoltage latch protection. By changing CDELAY, the
user can adjust the time delay between the output voltage
dropping below 80% of its nominal value and the part shut-
ting off due to undervoltage latch protection. The LDELAY
circuit consists ofa5µAcurrent source in series with a user
defined capacitor, CDELAY. The 5 µA current source is
turned on whenever the output voltage is below 80% of its
nominal value, otherwise this current source is off. With the
output voltage below 80% of its nominal value, the 5 µA
current source begins to charge CDELAY, as shown in Fig-
ure 2. If the potential across CDELAY reaches 2V, undervolt-
age latch protection will be enabled and the part will shut-
down. If the output voltage recovers to above 80% of its
nominal value before the potential across CDELAY reaches
2V, undervoltage latch protection will remain disabled.
Hence, CDELAY sets a time delay by the following equation:
T
DELAY
(ms) = C
DELAY
(nF) * 2V/5A
Undervoltage latch protection can be disabled by tying the
LDELAY pin to the ground.
COMPENSATION COMPONENTS
In the control to output transfer function, the first pole F
p1
can
be estimated as 1/(2πR
OUT
C
OUT
); The ESR zero F
z1
of the
output capacitor is 1/(2πESRC
OUT
); Also, there is a high
frequency pole F
p2
in the range of 45kHz to 150kHz:
F
p2
=F
s
/(πn(1−D))
whereD=V
OUT
/V
IN
, n = 1+0.348L/(V
IN
−V
OUT
)(LisinµHs
and V
IN
and V
OUT
in volts).
The total loop gain G is approximately 1000/I
OUT
where I
OUT
is in amperes.
A Gm amplifier is used inside the LM2655. The output resis-
tor R
o
of the Gm amplifier is about 80kΩ.C
c1
and R
C
together with R
o
give a lag compensation to roll off the gain:
F
pc1
= 1/(2πC
c1
(R
o
+R
c
)), F
zc1
= 1/2πC
c1
R
c
.
10128422
FIGURE 2. Undervoltage latch protection.
LM2655
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