SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 13

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

results from the ESR of the output capacitor while the
second term is due to the charging and discharging of
C
f = 1MHz and C
(7),
Depending on operating frequency and the type of
capacitor, ripple voltage resulting from charging and
discharging of C
10 F or 22 F X5R ceramic capacitor is found adequate
for output filtering in most applications. Ripple current
in the output capacitor is not a concern because the
inductor current of a buck converter directly feeds C
resulting in very low ripple current. Avoid using Z5U and
Y5V ceramic capacitors for output filtering because these
types of capacitors have high temperature and high
voltage coefficients.
Freewheeling Diode
Use of Schottky barrier diodes as freewheeling rectifiers
reduces diode reverse recovery input current spikes,
easing high-side current sensing in the SC2440. These
diodes should have a RMS current rating between 1A
and 2A and a reverse blocking voltage of at least 5V
higher than the input voltage. For switching regulators
operating at low duty cycles (i.e. low output voltage to
input voltage conversion ratios), it is beneficial to use
freewheeling diodes with somewhat higher RMS current
ratings (thus lower forward voltages). This is because the
diode conduction interval is much longer than that of
the transistor. Converter efficiency will be improved if
the voltage drop across the diode is lower.
The freewheeling diodes should be placed close to the
SW pins of the SC2440 to minimize ringing due to trace
inductance. Surface-mount equivalents of 1N5817 and
1N5819, MBRM120LT3 (ON Semi), UPS120 and
UPS140 (Micro-Semi) are all suitable.
Bootstrapping the Power Transistors
To maximize efficiency, the turn-on voltage across the
internal power NPN transistors should be minimized. If
these transistors are to be driven into saturation, then
their bases will have to be driven from a power supply
POWER MANAGEMENT
Applications Information
OUT
2005 Semtech Corp.
by the inductor ripple current. Substituting I
V
OUT
0
1
6 .
8 .
A
mV
OUT
OUT
3 (
m
may be higer than that due to ESR. A
= 10 F ceramic with ESR = 3m
7
5 .
mV
12
5 .
9
m
3 .
mV
)
L
= 0.6A,
OUT
in
,
13
higher in voltage than V
voltage (at least 2.5V higher than the SW voltage over
the industrial temperature range) is generated with a
bootstrap circuit (the diode D
Figure 8). The bootstrapped output (the common node
between D
the SC2440. The power transistor in the SC2440 is first
switched on to build up current in the inductor. When
the transistor is switched off, the inductor current pulls
the SW node low, allowing C
D
voltage goes high. This brings the BOOST voltage to
with each subsequent switching cycle, as does the
bootstrapped voltage at the BOOST pin. After a number
of switching cycles, C
approximately equal to that applied to the anode of D
Figure 7 shows the typical minimum BOOST to SW voltage
required to fully saturate the power transistor. This
differential voltage (
room temperature. This is also specified in the “Electrical
Characteristics” as “Minimum Bootstrap Voltage”. The
minimum required V
decreases. The bootstrap circuit reaches equilibrium
when the base charge drawn from C
on time is equal to the charge replenished during the off
interval.
V
BST
SW
Figure 7. Typical Minimum Bootstrap Voltage Re-
. When the power switch is again turned on, the SW
V
C
BST
, thus back-biasing D
BST
2.4
2.2
2.0
1.8
1.6
1.4
quired to Maintain Saturation at I
and C
-50
Minimum Bootstrap Voltage
BST
-25
BST
) is connected to the BOOST pin of
C BST
V
Temperature (°C)
vs Temperature
will be fully charged to a voltage
C
0
IN
BST
. The required driver supply
increases as temperature
) must be at least 1.8V at
BST
25
BST
BST
and the capacitor C
to be charged through
. C
50
BST
BST
75
voltage increases
during transistor
www.semtech.com
SC2440
100
SW
= 2A.
BST
BST
in
.