Applications Information: continued
discharge time is typically less than 10% of the charge
should conduct all the ripple current. RMS ripple current
can be as large as half the load current, and can be calcu-
lated as:
time. External components CT and RT allow the switching
frequency to be set by the user in the range between 10kHz
and 500kHz. CT can be chosen first based on size and cost
constraints. For proper operation over temperature, the
value of RT should be chosen within the range from 20k½
to 40k½. Any type of one-eighth watt resistor will be ade-
quate. Larger values of RT will decrease the maximum
duty cycle slightly. This occurs because the sink current on
the CT lead has an exponential relationship to the charge
current. Higher charge currents will discharge the CT lead
capacitor more quickly than lower currents, and a shorter
discharge time will result in a higher maximum duty
cycle.
V
OUT(VIN - VOUT)
IRIPPLE(RMS) = IOUT
2
VIN
Peak current requirement, load transients, ambient operat-
ing temperature and product reliability requirements all
play a role in choosing this component. Capacitor ESR and
the maximum load current step will determine the maxi-
mum transient variation of the supply voltage during
normal operation. The drop in the supply voltage due to
load transient response is given as:
Once the oscillator frequency and a value of CT have been
selected, the necessary value of RT can be calculated as fol-
lows:
ÆV = IRIPPLE(RMS) ´ ESR
1.88
RT =
(fOSC)(CT)
The type of capacitor is also an important consideration.
Aluminum electrolytic capacitors are inexpensive, but they
typically have low ripple current ratings. Choosing larger
values of capacitance will increase the ripple current
rating, but physical size will increase as well. Size con-
straints may eliminate aluminum electrolytics fro
where fOSC is the oscillator frequency in hertz, CT is given
in farads, and the value of RT is given in ohms. ESR effects
are negligible since the charge and discharge currents are
fairly small, and any type of capacitor is adequate for CT.
consideration. Aluminum electrolytics typically have
shorter operating life because the electrolyte evaporates
during operation. Tantalum electrolytic capacitors have
been associated with failure from inrush current, and man-
ufacturers of these components recommended derating the
capacitor voltage by a ratio 2:1 in surge applications. Some
manufacturers have product lines specifically tested to
withstand high inrush current. AVX TPS capacitors are
one such product. Ceramic capacitors perform well, but
they are also large and fairly expensive.
Selecting the Compensation Capacitor
As previously noted, the error amplifier does not con-
tribute greatly to transient response, but it does influence
noise immunity. The fast feedback loop input is compared
against the COMP pin voltage. The DC bias to the VFFB pin
may be provided directly from the output voltage, or
through a resistor divider if output voltage is greater than
2.9V. The desired percentage value of DC accuracy trans-
lates directly to the VFFB pin, and the minimum COMP pin
capacitor value can be calculated:
Startup
(16mA)(TOSC
)
CCOMP
=
(VFFBDC Bias Voltage)(tolerance)
At startup, output switching does not occur until two
undervoltage lockouts release. The first lockout monitors
the VIN lead voltage. No internal IC activity occurs until
VIN lead voltage exceeds the VIN turn-on threshold. This
threshold is typically 8.4V. Once this condition is met, the
on-chip reference turns on. As the reference voltage begins
to rise, a second undervoltage lockout disables switching
until VREF lead voltage is about 3.5V. The GATE leads are
held in a low state until both lockouts are released.
If fOSC = 200kHz, VFFB DC bias voltage is 2.8V and toler-
ance is 0.1%, CCOMP = 28.6µF. This is the minimum value
of COMP pin capacitance that should be used. It is a good
practice to guard band the tolerance used in the calcula-
tion. Larger values of capacitance will improve noise
immunity, and a 100µF capacitor will work well in most
applications.
As switching begins, the VFB lead voltage is lower than the
output voltage. This causes the error amplifier to source
current to the COMP lead capacitor. The COMP lead volt-
age will begin to rise. As the COMP lead voltage begins to
rise, it sets the threshold level at which the rising VFFB lead
voltage will trip the PWM comparator and terminate
switch conduction. This process results in a soft start inter-
val. The DC bias voltage on VFFB will determine the final
COMP voltage after startup, and the soft start time can be
approximately calculated as:
The type of capacitor is not critical, since the amplifier
output sink current of 16mA into a fairly large value or
wide range of ESR will typically result in a very small DC
output voltage error. The COMP pin capacitor also deter-
mines the length of the soft start interval.
Selecting the Input Bypass Capacitor
The input bypass capacitors minimize the ripple current in
the input supply, help to minimize EMI, and provide a
charge reservoir to improve transient response. The capac-
itor ripple current rating places the biggest constraint on
component selection. The input bypass capacitor network
V
FFB ´ CCOMP
TSOFT START
=
ICOMP(SOURCE)
10
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
