SC2604

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Applications Information

PWM Control Loop

The SC2604 is a voltage-mode PWM controller with a fixed

switching frequency of 400kHz for use in high efficiency,

boosted voltage, DC/DC power supplies.

As shown in Figure 2, the PWM control loop of the SC2604

consists of a 400kHz oscillator, a PWM comparator,

a voltage error amplifier, and a FET driver. The boost

converter output voltage is fed back to FB (error amplifier

negative) and is regulated to the reference voltage at

SS/VREF pin. The error amplifier output is compared

with the 400kHz ramp to generate a PWM wave, which

is amplified and used to drive the boost FET (Q

) for the converter. The PWM controller works with soft

start and fault monitoring circuitry to meet application

requirements.

UVLO, Start-up, and Shutdown

To initiate the SC2604, a supply voltage is applied to V

The DRV and GATE are held low. When V

UVLO (Under Voltage Lockout) threshold, typically 4.2V,

an internal current source (37µA) begins to charge the

OCP/EN pin capacitor. The OCP/EN voltage ramps from

near ground to over .25V but the voltage between

0.625V and .25V provides the linear soft-start range for

the disconnect FET (Q

over .25V, the OCP hiccup is enabled, and SS/VREF pin is

released. At this moment, another internal current source

(55µA) begins to charge the SS/VREF pin capacitor. When

the SS/VREF pin voltage reaches 0.5V, the error amplifier

output will rise to 0.4V, then the PWM comparator begins

to switch. The switching regulator output is slowly

ramping up for a soft turn-on. The details of SC2604 start-

up timing is shown in Figure 3.

If the supply voltage at V

(3.8V typically) during a normal operation, the DRV pin

is pulled low to cut off the supply power of the boost

converter, while the OCP/EN pin capacitor is discharged

with a µA internal current source. When the OCP/EN pin

falls below .25V, the SS/VREF pin is forced to ground. This

completely shuts down the boost conveter.

Directly pulling the OCP/EN pin below 0.52V can also

allow a complete shutdown of the output. Pulling the SS/

VREF pin below 0.V only shuts the boost FET (Q

) off and the output voltage will be (V

Figure 3. Start-up Timing Diagram

Hiccup Mode Short Circuit Protection

Hiccup mode over-current protection is utilized in the

SC2604. When an increasing load causes a voltage of

72mv to occur from V

sequence is started. The sequence starts by pulling DRV

low and discharging the OCP/EN pin with a µA current

source. When the OCP/EN pin falls below .25V, the SS/

VREF pin is forced to ground (similar to the UVLO shutdown

described in the last setion).

When the voltage on the OCP/EN pin falls to near zero

volt, the µA discharge current becomes a 37µA charging

current and the OCP/EN pin starts to charge and DRV is

enabled. When the OCP/EN voltage rises from 0.625V

to .25V, the current in the disconnect FET is allowed to

increase from zero to a maximum of 72mV/(Current Sense

Resistor Value). If the over-current condition still exists

when OCP/EN crosses .25V then the hiccup sequence

will re-start. If there is no over-current as OCP/EN crosses

.25V then the SS/VREF pin is released to rise and allow a