CS51411GMNR2G(2013) 데이터 시트보기 (PDF) - ON Semiconductor

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CS51411GMNR2G
(Rev.:2013)

1.5 A, 260 kHz and 520 kHz, Low Voltage Buck Regulators with External Bias or Synchronization Capability

ON Semiconductor 

CS51411, CS51412, CS51413, CS51414

independent of switching duty cycle. The maximum load

current, given by the following formula under continuous

conduction mode, is less than the Current Limit due to the

ripple current.

IO(MAX)

+

ILIM

*

VO(VIN * VO)

2(L)(VIN)(fs)

where:

fS = switching frequency,

ILIM = current limit threshold,

VO = output voltage,

VIN = input voltage,

L = inductor value.

When the regulator runs undercurrent limit, the

subharmonic oscillation may cause low frequency

oscillation, as shown in Figure 13. Similar to current mode

control, this oscillation occurs at the duty cycle greater than

50% and can be alleviated by using a larger inductor value.

The current limit threshold is reduced to Foldback Current

when the FB pin falls below Foldback Threshold. This

feature protects the IC and external components under the

power up or overload conditions.

Figure 13. The Regulator in Current Limit

BOOST Pin

The BOOST pin provides base driving current for the

power switch. A voltage higher than VIN provides required

headroom to turn on the power switch. This in turn reduces

IC power dissipation and improves overall system

efficiency. The BOOST pin can be connected to an external

boost−strapping circuit which typically uses a 0.1 mF capacitor

and a 1N914 or 1N4148 diode, as shown in Figure 1. When the

power switch is turned on, the voltage on the BOOST pin is

equal to

VBOOST + VIN ) VO * VF

where:

VF = diode forward voltage.

The anode of the diode can be connected to any DC voltage

other than the regulated output voltage. However, the

maximum voltage on the BOOST pin shall not exceed 40 V.

As shown in Figure 14, the BOOST pin current includes a

constant 7.0 mA predriver current and base current

proportional to switch conducting current. A detailed

discussion of this current is conducted in Thermal

Consideration section. A 0.1 mF capacitor is usually adequate

for maintaining the Boost pin voltage during the on time.

BIAS Pin (CS51412 and CS51414 Only)

The BIAS pin allows a secondary power supply to bias the

control circuitry of the IC. The BIAS pin voltage should be

between 3.3 V and 6.0 V. If the BIAS pin voltage falls below

that range, use a diode to prevent current drain from the

BIAS pin. Powering the IC with a voltage lower than the

regulator’s input voltage reduces the IC power dissipation

and improves energy transfer efficiency.

30

25

20

15

10

5

00

0.5

1.0

1.5

SWITCHING CURRENT (A)

Figure 14. The Boost Pin Current Includes 7.0 mA

Predriver Current and Base Current when the Switch

is Turned On. The Beta Decline of the Power Switch

Further Increases the Base Current at High

Switching Current

Shutdown

The internal power switch will not turn on until the VIN

pin rises above the Startup Voltage. This ensures no

switching until adequate supply voltage is provided to the

IC. The IC transitions to sleep mode when the SHDNB pin

is pulled low. In sleep mode, the internal power switch

transistor remains off and supply current is reduced to the

Shutdown Quiescent Current value (20 mA typical). This pin

has an internal pull-up current source, so defaults to high

(enabled) state when not connected.

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