SC2604EVB SEMTECH [Semtech Corporation], SC2604EVB Datasheet - Page 9

• Manufacturer Part Number: SC2604EVB
• Description: Simple PWM Boost Controller with Input Disconnect FET Drive
• Manufacturer: SEMTECH [Semtech Corporation]

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
© 2007 Semtech Corp.

). When the OCP/EN voltage is
IN
pin falls below UVLO threshold
IN
voltage exceeds
2
in Figure
IN
.
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
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
Figure 3. Start-up Timing Diagram
IN
to CS then a current limit hiccup
www.semtech.com
IN
-V
d
).
SC2604
2
in Figure
9