SC4508A データシートの表示（PDF） - Semtech Corporation

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SC4508A

Buck or Buck-Boost (Inverting) Current Mode Controller

Semtech Corporation 

SC4508A

POWER MANAGEMENT

Application Information (Cont.)

IL (pk) = IO

VIN + VO

VIN

+ VD

+

1

2

VIN

fs • L

(

VO + VD

VIN + VO + VD

)

for Buck − Boost

IO - full load current

VO - output voltage

VIN - input voltage

VD - diode forward voltage drop

fS - switching frequency

L - inductor

Error Amplifier

The error amplifier in the SC4508A is a transconductance

error amplifier, which is easily configured as a type two

compensator by connecting compensation network from

the COMP pin to AGND. The output voltage of the error

amplifier is compared to the sensed current signal

(amplified by gain 8) plus internal 500mV ramp to generate

duty cycle.

Both the non-inverting and the inverting inputs of the

error amplifier are brought out as device pins so that a

converter can be configured as either buck or buck-boost

converter.

Soft-Start and Overload Protection

The undervoltage lockout circuit discharges the SS/EN

capacitors. After VDD rises above 2.5V, the SS/EN

capacitors are slowly charged by internal 10uA current

source. As the SS/EN capacitor continues to be charged,

the VREF and the COMP voltage follows. The converter

gradually delivers increasing power to the output. The

inductor current follows the COMP voltage envelope until

the output goes into regulation.

After the SS/EN capacitor is charged above 1.4V (high

enough for the error amplifier to provide full load current),

the overload detection circuit is activated. If the CS pin

senses 32 consective switching cycles of over current,

the SC4508A will shut down and hold off the MOSFET

while discharging the soft-start capacitor. The SS/EN

capacitor is discharged with an internal 12mA current

sink. The overload latch is reset when the SS/EN capacitor

is discharged below 0.5V. The SS/EN capacitor is then

recharged with the 10µA current source and the converter

undergoes soft-start. If overload persists, the SC4508A

will undergo repetitive auto-shutdown-restart hiccup

mode.

In normal operation, the VREF voltage follows the SS/

EN voltage from 0 to 0.5V as the SS/EN voltage ramps

up from 1.4V to 1.9V. After the SS/EN voltage rises

above 1.9V, it settles at final value depending upon VDD.

If VDD higher than 7V, it is clamped around 6.8V. In the

worst case, the clamped voltage is about 10V. Therefore,

the external soft-start capacitor should be rated at least

16V.

The SS/EN pin can also be used as the enable input. The

MOSFET will be turned off if the SS/EN pin is pulled below

0.5V.

Converter Specifications

Buck or buck-boost converter design includes the

following specifications:

Input voltage range: Vin ∈ [Vin,min, Vin,max ]

Input voltage ripple (peak-to-peak): DVin

Output voltage: Vo

Output voltage accuracy: e

Output voltage ripple (peak-to-peak): DV

o

Nominal output (load) current: Io

Maximum output current limit: Io,max

Output (load) current transient slew rate: dIo (A/s)

Circuit efficiency: h

Selection criteria and design procedures for the

following are described.

1) output inductor (L) type and value

2) output capacitor (Co) type and value

3) input capacitor (Cin) type and value

4) power MOSFETs

5) current sensing and limiting circuit

6) voltage sensing circuit

7) loop compensation network

Inductor (L) and Ripple Current

The output inductor selection/design is based on the

output DC and transient requirements. Both output

current and voltage ripples are reduced with larger

inductors but it takes longer to change the inductor

current during load transients. Conversely smaller

inductors results in lower DC copper losses but the AC

 2007 Semtech Corp.

7

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