SC2620 datasheet(12/26 Pages) SEMTECH

12 / 26 page

 2006 Semtech Corp.

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SC2620

POWER MANAGEMENT

Applications Information

second term is due to the charging and discharging of

OUT by the inductor ripple current. Substituting ∆IL =

0.69A, f = 500kHz and C

OUT = 22µF ceramic with ESR =

Ω in (7),

)

(

OUT

Ω

⋅

∆

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

µF 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

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 CBST 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 DBST. When the power

switch is again turned on, the SW voltage goes high. This

brings the BOOST voltage to

BST

, thus back-biasing

BST. CBST 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 (

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

CBST 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.

Minimum Bootstrap Voltage

vs Temperature

1.4

1.6

1.8

2.0

2.2

2.4

-50

-25

100

Temperature (°C)

Figure 6. Typical Minimum Bootstrap Voltage Re-

quired to Maintain Saturation at I

SW = 2A.

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