SC2604
9
© 2010 Semtech Corp.
www.semtech.com
Applications Information
PWM Control Loop
The SC2604 is a voltage-mode PWM controller with a xed
switching frequency of 400kHz for use in high eciency,
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 amplier, and a FET driver. The boost
converter output voltage is fed back to FB (error amplier
negative) and is regulated to the reference voltage at
SS/VREF pin. The error amplier output is compared
with the 400kHz ramp to generate a PWM wave, which
is amplied and used to drive the boost FET (Q
2
in Figure
1) 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
IN
.
The DRV and GATE are held low. When V
IN
voltage exceeds
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 1.25V but the voltage between
0.625V and 1.25V provides the linear soft-start range for
the disconnect FET (Q
1
). When the OCP/EN voltage is
over 1.25V, 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 amplier
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
IN
pin falls below UVLO threshold
(3.8V typically) during a normal operation, the DRV pin
is pulled low to cut o the supply power of the boost
converter, while the OCP/EN pin capacitor is discharged
with a 1µA internal current source. When the OCP/EN pin
falls below 1.25V, 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 0.1V only shuts the boost FET (Q
2
in Figure
1) o and the output voltage will be (V
IN
-V
d
).
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
IN
to CS then a current limit hiccup
sequence is started. The sequence starts by pulling DRV
low and discharging the OCP/EN pin with a 1µA current
source. When the OCP/EN pin falls below 1.25V, 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 1µA 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 1.25V, 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 1.25V then the hiccup sequence
will re-start. If there is no over-current as OCP/EN crosses
1.25V then the SS/VREF pin is released to rise and allow a
V
IN
OCP/EN
SS/VREF
GATE
V
O
4.2V
T
2
1.25V+
1.25V
V
IN
+V
GS
DRV
0.625V
0.5V
T
1
V
IN
-V
d
Enable Hiccup
Note: T
1
=C
OCP/EN
*0.625V / 37µA
T
2
=C
SS/VREF
üüüüü µA
