SC2620
POWER MANAGEMENT
Applications Information
second term is due to the charging and discharging of
C
OUT
by the inductor ripple current. Substituting
∆I
L
=
0.69A, f = 500kHz and C
OUT
= 22µF ceramic with ESR =
2mΩ in (7),
their bases will have to be driven from a power supply
higher in voltage than V
IN
. The required driver supply
voltage (at least 2.5V higher than the SW voltage over the
industrial temperature range) is generated with a bootstrap
circuit (the diode D
BST
and the capacitor C
BST
in Figure 7).
The bootstrapped output (the common node between D
BST
and C
BST
) is connected to the BOOST pin of the SC2620.
The power transistor in the SC2620 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
BST
to be charged through D
BST
. When the power
switch is again turned on, the SW voltage goes high. This
brings the BOOST voltage to
V
SW
+
V
C
BST
, thus back-biasing
D
BST
. C
BST
voltage increases with each subsequent switching
cycle, as does the bootstrapped voltage at the BOOST pin.
After a number of switching cycles, C
BST
will be fully charged
to a voltage approximately equal to that applied to the
anode of D
BST
. Figure 6 shows the typical minimum BOOST
to SW voltage required to fully saturate the power transistor.
This differential voltage (
=
V
C
BST
) must be at least 1.8V at
room temperature. This is also specified in the “Electrical
Characteristics” as “Minimum Bootstrap Voltage”. The
minimum required V
C BST
increases as temperature
decreases. The bootstrap circuit reaches equilibrium when
the base charge drawn from C
BST
during transistor on time
is equal to the charge replenished during the off interval.
∆
V
OUT
=
0.69 A
⋅
(2m
Ω +
11.4m
Ω
)
=
1.4mV
+
7.8mV
=
9.2mV
Depending on operating frequency and the type of
capacitor, ripple voltage resulting from charging and
discharging of C
OUT
may be higer than that due to ESR. A
10µF to 47µF 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
OUT
, 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 SC2620. These diodes
should have an average forward current rating between
1A and 2A and a reverse blocking voltage of at least a few
volts 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 average 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 SC2620 to minimize ringing due to trace
inductance. 10BQ015, 20BQ030 (International Rectifier),
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
2006 Semtech Corp.
12
Minimum Bootstrap Voltage
vs Temperature
2.4
2.2
Voltage (V)
2.0
1.8
1.6
1.4
-50
-25
0
25
50
75
100
Temperature (°C)
Figure 6. Typical Minimum Bootstrap Voltage Re-
quired to Maintain Saturation at I
SW
= 2A.
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