ISL6719
7
FN6555.2
July 15, 2014
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COMPA, COMPB
A compensating capacitor is placed between COMPA and
COMPB to stabilize the control loop. The values may vary
depending on the output load and capacitance applied between
VSW and GND, but for all applications having a 1.0µF load
capacitor, a 220pF compensation capacitor is recommended.
The voltage at COMPA is nominally 0.7V. The voltage at COMPB
is nominally VSW +5.0V.
Functional Description
Features
The control circuitry used in Telecom/Datacom DC/DC
converters typically requires an operating bias voltage
significantly lower than the source voltage available to the
converter. Many applications use a discrete linear regulator from
the input source to create the bias supply. Often an auxiliary
winding from the power transformer is used to supplement or
replace the linear supply once the converter is operating. The
auxiliary winding bias voltage may require regulation as well to
minimize the voltage variation inherent in slave windings. When
implemented discretely, this circuitry occupies significant PWB
area, a considerable problem in today’s high density converters.
The ISL6719 linear regulator simplifies the start-up and
operating bias circuitry needed in Telecom and Datacom DC/DC
converters by integrating these functions, and more, in a small
3mm x 3mm DFN package.
AUXIN
AUXIN is the auxiliary input of the ISL6719, accepting bias
voltage whenever the input source voltage, VPWR, is above its
undervoltage lockout (UVLO) threshold. The VSW selects AUXIN
as its source when it is capable supporting the load on VSW.
Otherwise VPWR is selected.
AUXIN can accept voltages up to 40V maximum. Voltages in
excess of 40V, including transients, will cause permanent
damage to the device. Care should be taken when connecting
external sources through very long traces or lead wires. The
lead inductance may cause unexpected transients in excess of
the device’s ratings. In such circumstances it is recommended
that a small resistor be placed between AUXIN and the
external source to dampen the transient. A value of 10 to
100 is usually sufficient.
VSW
The VSW is the switched output and may be turned on and off
using the ENABLE or ENABLE_N pins. The VSW is adjustable
from 1.5V to 20V, but must always be at least 6.2V lower than
VPWR at rated load. Additionally, VSW must be at least 3.0V
lower than AUXIN for it to function as the source for VSW. As the
differential voltage between AUXIN and VSW drops below 3.0V,
the input current will shift from AUXIN to VPWR. The voltage
headroom required is load dependent (see Figures 1 and 2). The
VSW preferentially uses AUXIN as its input source, but if AUXIN
is unable to supply adequate voltage, VPWR is selected as the
alternate input source. The VSW is capable of delivering up to
100mA continuously, depending on power dissipation and the
thermal environment in which the device is placed.
The output voltage is adjusted using the VSW_FB input. The
VSW is set with a resistor divider from VSW to ground with the
central node connected to VSW_FB. Refer to Figure 5.
Referring to Equation 1, the V
REF
is nominally 1.5V and I
BIAS
has a maximum value of 1.5µA. The error introduced by the
VSW_FB bias current can be minimized by making the product
of R
1
x I
BIAS
small, relative to the magnitude of the desired
output voltage. For example, setting R
1
x I
BIAS
equal to 0.5%
of VSW yields a value for R
1
equal to 3.33 x VSW (k).
The VSW requires an external compensation capacitor to
remain stable across the output adjustment range, output
capacitance and loading. A value of 220pF between COMPA
and COMPB is recommended for all operating conditions with
a nominal load capacitance of 1.0µF (0.47µF to 1.5µF). The
VSW requires a minimum load of 3mA.
VSW V
REF
R
1
R
2
+
R
2
---------------------
I
BIAS
R
1
–= V
(EQ. 1)
C2
1.0F
1
VSW_FB
VPWR
COMPB
VSW
AUXIN
2
5
7
8
9
3
4
6
COMPA
ENABLE
ENABLE_N
GND
R
1
R
2
VSW
C3
220pF
FIGURE 5. VSW ADJUSTMENT AND COMPENSATION
