0.5A, 300kHz, SO-8 PWM/PFM Step Down Converter with Shutdown
SD1
V
1
2
8
7
IN
+
*CIN
L
22µH
ILC6377
(TOP VIEW)
10µF
S/D
3
4
6
5
VO UT
+
CSS
CL
47µF
Fig. 1: Typical step-down wn DC-DC
Fig 1. Typical step-down DC-DC
ccoonnvverrtterr aappppliiccaattioonn
SD1 : MA735 Schottky Diode ( MATSUSHITA )
CL : 10V/47µF Tantalum Capacitor ( NICHICON, F93 )
CSS : 4700pF Ceramic Capacitor
CIN : 16V / 10µF Tantalum Capacitor ( NICHICON, F93 )
Figure 2 shows a typical fixed output voltage step-down DC-DC
converter application circuit for ILC6377SO-XX
Over-current and short-circuit protection
External component selection
In the event of an over-current or short-circuit condition, the
ILC6377 cycles the soft-start pin in a hiccup mode to provide
fault protection. When the output voltage decreases due to
overload, the ILC6377 will operate continuously at the max-
imum duty cycle. If the period of maximum duty cycle oper-
ation exceeds TPRO (typically 5 msec), pin 4 will be pulled
low; thus discharging the external soft-start capacitor CSS.
This action inhibits the regulator’s PWM action. Next, the
ILC6377’s soft-start circuitry starts recharging CSS and initi-
ates a controlled start-up. If the overload condition continues
to exist, the above sequence of events will repeat; thus con-
tinuing to cycle the soft-start function.
Proper selection of external components is important for
achieving high performance. The output inductor selected
should have low DC resistance on the order of 0.2W or less
and saturation current rating of 1A or higher.
Recommended inductors are Sumida CD54 (22mH, 0.18W
max DC resistance) or Coilcraft DO3308P-223 (22mH,
0.18W max DC resistance) or equivalent.
The catch diode should be a schottky diode with low for-
ward drop and rated at 1A or greater current, MA735 or it’s
equivalent is recommended.
Input and output capacitors should be tantalum capacitors
with low equivalent series resistance (ESR) and voltage rat-
ing higher than the actual application.
Note that very little power is dissipated with this method of
fault protection versus constant current limit protection.
Even though the internal power MOSFET is pulsed on and
off at high peak current, the DC current is low; thus leading
to low power dissipation even under short-circuit conditions.
Soft-start
Pin 4 of ILC6377 functions as the soft-start pin as well as the
shutdown pin. A soft-start capacitor (from pin 4 to ground)
controls the rate at which the power supply starts up; thus
preventing large overshoots at the output as well as large in-
rush current. The value for CSS should be 100pF or greater.
Keep in mind that the duration of maximum duty cycle
condition is used to trigger the ILC6377’s fault protec-
tion circuit. As such, a small input-output (VIN - VOUT
)
differential voltage may trigger the device’s fault protec-
tion circuitry even at low output current.
Shutdown
The ILC6377 is placed in shutdown mode by taking pin 4 to
ground. In shutdown, the quiescent current of the device is
under 2mA. When using the shutdown feature, pin 4 must
be driven from an open collector or open drain output with-
out employing an external pull-up resistor, as shown in fig-
ure 2. Since pin 4 is also used to charge an external capac-
itor for soft-start, this pin should not be driven from a push-
pull CMOS type output.
Undervoltage Lockout
The undervoltage lockout feature prevents faulty operation
by disabling the operation of the regulator when input volt-
age is below the minimum operating voltage, VUVLO. When
the input voltage is lower than VUVLO the device disables
the internal P-channel MOSFET and provides “high” output
at both EXT1 and EXT2 outputs.
Impala Linear Corporation
(408) 574-3939
ILC6377 1.3
www.impalalinear.com
June 1999
5
IMPALA [ Impala Linear Corporation ]
