SC1480
POWER MANAGEMENT
Applications Information
RTON is a resistor connected from the input supply to the
TON pin. The graph on page 19 shows the relationship
between RTON and switching frequency.
+5V Bias Supply
The SC1480 requires an external +5V bias supply in ad-
dition to the battery. If stand-alone capability is required,
the +5V supply can be generated with an external linear
regulator. There are two inputs for the external +5V bias
supply, VCCA & VDDP. The VCCA input powers the analog
section of the SC1480 while the VDDP input provides
power to the upper and lower gate drivers. VCCA will need
to be decoupled from the +5V supply through a 10 Ohm
resistor and the addition of a filter capacitor from VCCA
to ground. VCCA and VDDP must be separate in order to
utilize the low power S3 state of the SC1480. The bat-
tery input VIN and the +5V input VCCA can be tied to-
gether if the input voltage is fixed from +4.5V to +5.5V;
however, as before, VCCA will need to be decoupled from
the +5V supply through a 10 Ohm resistor and the addi-
tion of a filter capacitor from VCCA to ground.
Reference I/O
The reference input can be generated off of a 2.5V or
VDDQ supply by a simple resistive divider. Resistors less
than 100k Ohms should be used and a small filter ca-
pacitor from the reference input to ground of 0.1uF will
remove any ripple voltage present on the input. The in-
put has a common mode range of the REFIN threshold
to 2.5V.
The voltage on the reference input passes through a unity
gain buffer prior to being sent to REFOUT. This reference
output has a class A output stage with 3mA of sourcing
capacity. It has a pull-down impedance of 50k. The out-
put will require a small RC filter of 10 Ohms and 1µF to
maintain stability.
Pseudo-Fixed Frequency Constant On-Time PWM
Controller
Shutdown, Suspend to RAM and Run Mode
The SC1480 has three modes of operation: shutdown,
suspend to RAM, and run mode. All three modes must
have VCCA connected at all times. Shutdown mode is
controlled by REFIN. When REFIN is below 0.8V, the ref-
erence buffer will be off and the SMPS is disabled. In
this mode the bias current of the device will be less than
10µA.
The PWM control architecture consists of a constant-on-
time, pseudo fixed frequency PWM controller, (Figure 1).
The output ripple voltage developed across the output
filter capacitors ESR provides the PWM ramp signal elimi-
nating the need for a current sense resistor. The high-
side switch on-time is determined by a one-shot whose
period is directly proportional to output voltage and in-
versely proportional to input voltage. A second one-shot
sets the minimum off-time to 400ns typically.
Suspend to RAM, or S3 State, is controlled by REFIN and
VDDP. With REFIN is above 0.8V and VDDP is low (below
3V), the device will output the reference voltage onto
REFOUT, but the SMPS is disabled. In this mode the bias
current is approximately 125µA.
Run mode is activated by maintaining REFIN above 0.8V
and VDDP above 3V. In this mode the reference and SMPS
are active.
On-Time One-Shot (TON)
The on-time one-shot comparator has two inputs. One
input looks at the output voltage, while the other input
samples the input voltage and converts it to a current.
This input proportional current is used to charge an in-
ternal on-time capacitor. The TON time is the time re-
quired for the voltage on the capacitor to charge from
zero volts to VOUT, thereby making the on-time of the
high-side switch directly proportional to output voltage
and inversely proportional to input voltage. This imple-
mentation results in a nearly constant switching frequency
without the need of a clock generator.
Current Limit Circuit
Current limiting of the SC1480 can be accomplished in
two ways. First, the device can implement on-state re-
sistance of the low-side MOSFET as the current sensing
element (RDSON sensing). Second, the device can accept
a resistive element in the low-side source (RSENSE, resistor
sensing). The second method offers greater accuracy of
the current limit threshold over RDSON sensing, at the
added expense of a sense resistor and associated effi-
ciency loss.
VOUT
TON =3.3x10−12 •(RTON+37x103 )•
+50ηS
V
IN
Whether RDSON sensing or RSENSE resistor sensing is used,
2006 Semtech Corp.
7
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