SC4525DEVB datasheet(9/21 Pages) SEMTECH

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SC4525D

Applications Information

Operation

The SC4525D is a 350kHz fixed frequency, peak current-

mode, step-down switching regulator with an integrated

3.9A power NPN transistor. With the peak current-mode

control, the double reactive poles of the output LC filter

are reduced to a single real pole by the inner current

loop. This simplifies loop compensation and achieves fast

transient response with a simple Type-2 compensation

network.

As shown in Figure 2, the switch collector current is

sensed with an integrated 3.53mW sense resistor. The

sensed current is summed with a slope-compensating

ramp before it is compared with the transconductance

error amplifier (EA) output. The PWM comparator trip

point determines the switch turn-on pulse width. The

current-limit comparator ILIM turns off the power switch

when the sensed signal exceeds the 18mV current-limit

threshold.

Driving the base of the power transistor above the

input power supply rail minimizes the power transistor

saturation voltage and maximizes efficiency. An external

bootstrap circuit (formed by the capacitor C

1 and the

diode D

1 in Figure 1) generates such a voltage at the BST

pin for driving the power transistor.

Shutdown and Soft-Start

The SS/EN pin is a multiple-function pin. An external

capacitor (4.7nF to 22nF) connected from the SS pin to

ground sets the soft-start and overload shutoff times of

the regulator (Figure 3). The effect of V

SS/EN on the SC4525D

is summarized in Table 1.

Table 1: SS/EN operation modes

Pulling the SS/EN pin below 0.2V shuts off the regulator

and reduces the input supply current to 18µA (V

IN = 5V).

When the SS/EN pin is released, the soft-start capacitor

SS/EN

Mode

Supply Current

<0.2V

Shutdown

18uA @ 5Vin

0.4V to 1.2V

Not switching

2mA

1.2V to 2.15V

Switching & hiccup disabled

>2.15V

Switching & hiccup armed

Load dependent

SS/EN

Mode

Supply Current

<0.2V

Shutdown

18uA @ 5Vin

0.4V to 1.2V

Not switching

2mA

1.2V to 2.15V

Switching & hiccup disabled

>2.15V

Switching & hiccup armed

Load dependent

is charged with an internal 1.9µA current source (not

shown in Figure 3). As the SS/EN voltage exceeds 0.4V,

the internal bias circuit of the SC4525D turns on and the

SC4525D draws 2mA from V

IN. The 1.9µA charging current

turns off and the 2.4µA current source I

C in Figure 3 slowly

charges the soft-start capacitor.

The error amplifier EA in Figure 2 has two non-inverting

inputs. The non-inverting input with the lower voltage

predominates. One of the non-inverting inputs is biased

to a precision 1V reference and the other non-inverting

input is tied to the output of the amplifier A

1. Amplifier A1

produces an outputV

1 = 2(VSS/EN -1.2V). V1 is zero and COMP

is forced low when V

SS/EN is below 1.2V. During start up,

the effective non-inverting input of EA stays at zero until

the soft-start capacitor is charged above 1.2V. Once V

SS/

EN exceeds 1.2V, COMP is released. The regulator starts to

switch when V

COMP rises above 0.4V. If the soft-start interval

is made sufficiently long, then the FB voltage (hence the

output voltage) will track V

1 during start up. VSS/EN must be

at least 1.83V for the output to achieve regulation. Proper

soft-start prevents output overshoot. Current drawn from

the input supply is also well controlled.

Overload / Short-Circuit Protection

Table 2 lists various fault conditions and their

corresponding protection schemes in the SC4525D.

Table 2: Fault conditions and protections

As summarized in Table 1, overload shutdown is disabled

during soft-start (V

SS/EN<2.15V). In Figure 3, the reset input

of the overload latch B

2 will remain high if the SS/EN

voltage is below 2.15V. Once the soft-start capacitor is

charged above 2.15V, the output of the Schmitt trigger

1 goes high, the reset input of B2 goes low and hiccup

becomes armed. As the load draws more current from

Condition

Cause of Fault

Protective Action

IL>ILimit, VFB>0.8V

Over current

Cycle-by-cycle limit at

programmed frequency

IL>ILimit, VFB<0.8V

Over current

Cycle-by-cycle limit with

frequency foldback

VSS/EN Falling

SS/EN<1.9V

Persistent over current

or short circuit

Shutdown, then retry

(Hiccup)

Tj>160C

Over temperature

Shutdown

Condition

Cause of Fault

Protective Action

IL>ILimit, VFB>0.8V

Over current

Cycle-by-cycle limit at

programmed frequency

IL>ILimit, VFB<0.8V

Over current

Cycle-by-cycle limit with

frequency foldback

VSS/EN Falling

SS/EN<1.9V

Persistent over current

or short circuit

Shutdown, then retry

(Hiccup)

Tj>160C

Over temperature

Shutdown

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