sc120skevb Semtech Corporation, sc120skevb Datasheet - Page 23

• Manufacturer Part Number: sc120skevb
• Description: Low Voltage Synchronous Boost Regulator
• Manufacturer: Semtech Corporation

Applications Information (continued)
accumulates with each successive cycle in the burst. The
result is that the output load current that can be sup-
ported in PSAVE under this high V
greater than occurs if the 240mA current limit can be
enforced. Therefore the PSAVE exit load due to the fi rst
exit criterion (Figure 2) can increase significantly. This
phenomenon is advantageous. Reverting to PWM opera-
tion with high V
to the PWM minimum duty cycle. PSAVE operation avoids
this voltage rise because of its hysteretic voltage-thresh-
old on/off control. If the load remains low enough to
remain in PSAVE, V
exceed V
startup load must be small enough to cause discontinu-
ous mode PWM operation. This PSAVE mode startup load
upper limit can be increased if needed by reducing the
inductance. (Refer to the Inductor Selection section.)
Suffi ciently large output capacitance will prevent PSAVE
exit due to the second exit criterion (Figure 3).
PSAVE V
tance on the external FB pin network. If using external
feedback programming, it is prudent to add a small capaci-
tor between OUT and FB to the circuit board layout. When
operating the SC120 in the fi nal confi guration in PSAVE,
observe the amplitude of PSAVE ripple. If the ripple
exceeds 50mV for the expected range of input voltage, a
small-value capacitor should be tried. Capacitance on the
order of a few picofarads is often suffi cient to bring the
ripple amplitude to approximately 50mV.
In the case of low V
may be needed, perhaps 4.7pF or higher. If using the
SOT23-6 package (SC120SKTRT) with low V
at least 10pF to 12pF is recommended.
The Enable Pin
The EN pin is a high impedance logical input that can be
used to enable or disable the SC120 under processor
control. V
a high-impedance state (turn off both FET switches), and
turn on an active discharge device to discharge the output
capacitor via the OUT pin. V
output. The startup sequence from the EN pin is identical
to the startup sequence from the application of input
power.
OUT
OUT
EN
ripple may increase due to parasitic capaci-
< 0.2V will disable regulation, set the LX pin in
. To initally enter PSAVE mode, the initial
IN
can result in V
IN
IN
and high V
can approach and even slightly
EN
OUT
> 0.85V will enable the
OUT
rising above V
, larger values of CFB
IN
condition will be
IN
and high V
REG
, due
OUT
,
Regulator Startup, Short Circuit Protection, and
Current Limits
The SC120 permits power up at input voltages from 0.85V
to 4.5V. Startup current limiting of the internal switching
n-channel and p-channel FET power devices protects
them from damage in the event of a short between OUT
and GND. As the output voltage rises, progressively less-
restrictive current limits are applied. This protection
unavoidably prevents startup into an excessive load.
To begin, the p-channel FET between the LX and OUT pins
turns on with its current limited to approximately 150mA,
the short-circuit output current. When V
(but is still below 1.7V), the n-channel current limit is set
to 350mA (the p-channel limit is disabled), the internal
oscillator turns on (approximately 200kHz), and a fi xed
75% duty cycle PWM operation begins. (See the section
PWM Operation.) When the output voltage exceeds 1.7V,
fixed frequency PWM operation begins, with the duty
cycle determined by an n-channel FET peak current limit
of 350mA. When this n-channel FET startup current limit
is exceeded, the on-state ends immediately and the off -
state begins. This determines the duty cycle on a cycle-
by-cycle basis. When V
regulation voltage, the n-channel FET current limit is
raised to 1.2A, and normal voltage regulation PWM control
begins.
Once normal voltage regulation PWM control is initiated,
the output becomes independent of V
lation can be maintained for V
the maximum duty cycle and peak current limits. The duty
cycle must remain between 15% and 90% for the device
to operate within specifi cation.
Note that startup with a regulated active load is not the
same as startup with a resistive load. The resistive load
output current increases proportionately as the output
voltage rises until it reaches programmed V
a regulated active load presents a constant load as the
output voltage rises from 0V to programmed V
also that if the load applied to the output exceeds an
applicable V
cycle limit, the criterion to advance to the next startup
stage may not be achieved. In this situation startup may
OUT
dependent startup current limit or duty
OUT
is within 2% of the programmed
IN
as low as 0.7V, subject to
IN
OUT
and output regu-
approaches V
OUT
/R
SC120
LOAD
OUT
. Note
, while
23
IN