ISL6567_07 Datasheet, PDF (13/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
The output voltage can be set via a simple resistor divider, as
shown in Figure 8. It is recommended this resistor divider is
connected at the input of the differential amplifier (as this
amplifier is powered from the IC’s 5V bias and has limited
input range). To avoid degradation of DC regulation tolerance
R_P
=
R
⋅
-----------^V-----^O----^U----^T-------------
V_OUT – V_REF
R_S
=
R ⋅ ^V-----^O----^U----^T--
V_REF
due to the differential amplifier’s input resistance, a size
requirement is placed on the combined value of R_P and R_S.
Consider R to be the parallel combination of these two resis-
tors, and use a value of 2kΩ or less for R; use the following
equations to determine the value of R_P and R_S, based on the
desired output voltage, the reference voltage, and the chosen
value of R.
V_REF
ISL6567
+
ERROR ^-
AMPLIFIER
EXTERNAL CIRCUIT
FB
R_S(R₁)
+
To V_OUT
R_P
VDIFF
+
X1
-
DIFFERENTIAL
AMPLIFIER
VSEN
RGND
FIGURE 9. SETTING THE OUTPUT VOLTAGE AT THE FB PIN
The differential amplifier can be used even if remote output
sensing is not desired or not feasible, simply connect RGND
to the local ground and connect VSEN to the output voltage
being monitored. Should one desire to bypass the differential
amplifier, the circuit in Figure 9 is recommended as the
proper implementation. Since its output is monitored for OVP
and PGOOD purposes, the differential amplifier needs to be
connected to the feedback circuit at all times, hence its input
connections to FB and local ground. However, its output,
VDIFF, can be left open. The resistor divider setting the out-
put voltage is calculated in a manner identical to that already
revealed.
-
+
DIFFERENTIAL
AMPLIFIER
ISL6567
EXTERNAL
CIRCUIT
Z_IN
~2μA
VDIFF
VSEN
RGND
Z_IN > 240kΩ
over process, temperature, and
0 < VSEN-RGND < 2.5V
FIGURE 10. SETTING THE OUTPUT VOLTAGE AT THE INPUT
OF THE DIFFERENTIAL AMPLIFIER
DIFFERENTIAL AMPLIFIER’S UNITY GAIN NETWORK
The differential amplifier on the ISL6567 utilizes a typical
resistive network along with active compensating circuitry to
set its unity gain. This resistive network can affect the DC
regulation setpoint in proportion to its relative magnitude
compared to the external output voltage setting resistor
divider. Figure 10 details the internal resistive network. For
minimal impact on the output voltage setting, follow the
guidelines presented in the Setting the Output Voltage
paragraph.
EXTERNAL REFERENCE OPERATION
The ISL6567 is capable of accepting an external voltage and
using it as a reference for its output regulation. To enable this
mode of operation, the MON pin potential has to be below
3.5V and the reference voltage has to be connected to the
REFTRK pin. The internal or external reference mode of
operation is latched in every time the POR is released or the
ISL6567 is enabled. The highest magnitude external
reference fed to the REFTRK pin that the ISL6567 can follow
is limited to 2.3V. The ISL6567 utilizes a small initial negative
offset (typically about 50mV) in the voltage loop at the
beginning of it soft-start, to counteract any positive offsets
that may have undesirable effects. As this initial offset is
phased out as the reference is ramped up to around 200mV,
in order to avoid an error in the output regulation level, it is
recommended the external reference has an amplitude
(final, DC level) exceeding 300mV.
13
FN9243.2
March 20, 2007