SC4620 datasheet(12/19 Pages) SEMTECH

12 / 19 page

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POWER MANAGEMENT

SC4620

Operation

Application Information (Cont.)

Synchronization Frequency

Synchronization operation mode is invoked by using an

external clock signal and is activated when the SYNC/EN

is pulled and held above 2V and held below 0.8V. The

range of synchronization frequency is from 200kHz to

2MHz.

A jitter happens when sync pulse clock edge is less than

120ns before the phase switches. It is caused by the

ground bounce of synchronization pulse coupled to PWM

comparator. Users try to avoid this application. (refer to

Figure 9).

Power Good Indicator

The PGOOD pin is an open-drain and incorporated window

comparators output. It’s is necessary that a pull-up resis-

tor from the PGOOD pin to the input supply for setting the

logic high level of the PGOOD signal. When FB voltage is

within +10% setting output voltages typical, the output of

power good comparator becomes high impedance after

delay time. The PGOOD signal delay time is around 1024/

OSC. In shutdown mode the power good output is actively

pulled low.

For example, 1MHz switching frequency applications, the

PGOOD delay time is around 1ms.

Thermal Shutdown

When the junction temperature rises up around 160°C,

the internal soft start voltage is held low, the internal high

side and low side MOSFETs are turned off and the output

voltage will fall to zero. Once the junction temperature

goes below hysteresis temperature around 10°C, the

regulator will restart. (refer to Figure 8).

Linear Mode Operation (100% duty)

The SC4620 can allows 100% duty cycle operation. The

Vout is,

Transient response

Test condition: 3.3Vin, 1.2Vo, Io=0 to 3A

R=F=2.5A/us,T1=T2=0.3ms

Vout

where

L : Output inductor DC resistance.

DSH : Internal high side P-MOSFET resistance.

(refer to Figure11).

As Vin drops gradually and close to Vout, the buck regulator

will go into 100% duty cycle ratio. A matter needing

attention is internal high side PMOS has minimum off time

limitation and is related to duty cycle rate. This condition

makes the working duty cycle perform at randon with the

output ripple increasing and a poor transient response.

Above phenomenon can be improved by larger output

capacitor and smaller output inductor. Users need to

verify whether above application condition has opposite

influence on entire circuit.

Over Current Protection

A over current setting is programmed by an external

resistor (R

ISET). It goes through internal sense resistor and

generates a voltage.

where

I : The current is generated by R

ISET , and it is amplified

by internal current amplifier.

ONSENSE : Internal sense resistor.

Output inductor current goes through internal high side

P-MOSFET and generate a voltage.

where

L : Output inductor current.

DSH(ON) : High side P-MOSFET conduction resistance.

After the high side PMOS turn on around 30ns, the OCP

comparator will compare between V2 and V1. When the

converter detects an over current condition (V2 > V1) as

shown in Figure 16, the SC4620 proceeds into the cycle by

cycle protection mode (Point B to Point C), which responds

to minor over current cases and the output voltage is

monitored.

If the over current and low output voltage (set at 60% of

nominal output voltage) occur at the same time, the SS

pin is pull low by an internal switch and the comp pin is

pulled low and the devices stops switching. Assume start

from FB = 0V, FB and SS voltage rise forward 0.5V. Once

SS voltage exceeds 0.4V, the hiccup comparator becomes

enabled. The hiccup period is around 217/F

OSC. (Point C to

Point D).

For example, with a switching frequency application of

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