SC4524SETRT 查看數據表（PDF） - Semtech Corporation

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SC4524SETRT

Programmable Frequency, 2A Output 30V Step-Down Switching Regulator

Semtech Corporation 

SC4524

POWER MANAGEMENT

Applications Information

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.

during the off interval.

The switch base current = ,6:

+

,6: , where ISW and b

are the switch emitter current and current gain

respectively, is drawn from the bootstrap capacitor CBST.

Charge ,6:721 is drawn from CBST during the switch on

The freewheeling diodes should be placed close to the

SW pins of the SC4524 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 transistor should be minimized. If

the transistor is to be driven into saturation, then its

base will have to be driven from a power supply higher in

voltage than VIN. 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

time, resulting in

a

voltage

droop

of

,6: 721

&%67

. If ISW

=

2A,

TON = 1ms, b = 35 and CBST = 0.1mF, then the 9&%67 droop

will be 0.57V. CBST is refreshed to 9$ - 9'%67 + 9'5(&7 every

cycle, where 9$ is the applied DBST anode voltage. Switch

base current discharges the bootstrap capacitor to

9$

- 9'%67

+

9'5(&7

-

,6: 721

b&%67

at the end of conduction. This

voltage must be higher than the minimum shown in Figure

6 to ensure full switch enhancement. DBST can be tied

either to the input or to the output of the DC/DC

converter.

(the diode DBST and the capacitor CBST in Figure 7). The

bootstrapped output (the common node between DBST

and CBST) is connected to the BST pin of the SC4524.

The power transistor in the SC4524 is first switched on

to build up current in the inductor. When the transistor

is switched off, the inductor current pulls the SW node

If DBST is tied to the input, then the charge drawn from

the input power supply will be

,6: 721

b

(the base charge

of the switch). The energy loss due to base charge per

low, allowing CBST to be charged through DBST. When the

power switch is again turned on, the SW voltage goes

high. This brings the BST voltage to 96: + 9&%67 , thus back-

biasing DBST. CBST voltage increases with each subsequent

switching cycle, as does the bootstrapped voltage at the



H vv€ˆ€Ã7‚‚‡†‡…hƒÃW‚y‡htr

É †ÃUr€ƒr…h‡ˆ…r



BST pin. After a number of switching cycles, CBST will be



fully charged to a voltage approximately equal to that

ÃW 

r

t

applied to the anode of DBST. Figure 6 shows the typical

y‡h

‚ 

minimum BST to SW voltage required to fully saturate

W

the power transistor. This differential voltage ( = 9&%67 )

must be at least 1.8V at room temperature. This is also

specified in the “Electrical Characteristics” as “Minimum

Bootstrap Voltage”. The minimum required VCBST increases

as temperature decreases. The bootstrap circuit reaches

equilibrium when the base charge drawn from CBST during

transistor on time is equal to the charge replenished

















Ur €ƒr …h ‡ˆ…rÃ8



Figure 6. Typical Minimum Bootstrap Voltage Re-

quired to Maintain Saturation at ISW = 2A.

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