ISL6567_07 Datasheet, PDF (18/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
7
6
5
4
3
2
1
0
6
7
8
9
10
11
V_INmin (V)
FIGURE 19. NORMALIZED RESISTOR VALUE IN PASSIVE
CONFIGURATION; V_VCC = 5V
Figure 20 details the normalized maximum power dissipation
R_BIAS will be subject to in the given application. To use the
graph provided, find the power dissipation level
corresponding to the minimum input voltage and the input
voltage range and multiply it by the maximum desired bias
current to obtain the maximum power R_BIAS will dissipate.
150
ΔV_IN = 1V
135
ΔV_IN = 2V
120
ΔV_IN = 3V
ΔV_IN = 4V
105
ΔV_IN = 5V
ΔV_IN = 6V
90
ΔV_IN = 7V
75
ΔV_IN = 8V
60
45
30
15
0
6
7
8
9
10
11
V_INmin (V)
FIGURE 20. NORMALIZED RESISTOR POWER DISSIPATION
(AS SELECTED IN FIGURE 17) vs MINIMUM
INPUT VOLTAGE; V_VCC = 5V
Alternately, the maximum power dissipation inside R_BIAS
can be calculated using the following equation:
P_MAXRBIAS = (V_INMAX – V_VCC) ⋅ I_BIAS
Maximum power dissipation in the bias resistor will take
place at the upper end of the input voltage range. Select a
resistor with a power dissipation rating above the calculated
value and pay attention to design aspects related to the
power dissipation level of this component. Although Figures
18 through 20 assume a VCC voltage of 5V, the design aid
curves can be translated to a different VCC voltage by
translating them in the amount of the voltage differential, to
the left for a lower VCC voltage, or to the right for a higher
VCC voltage,
Should the simple series bias resistor configuration fall short
of providing the necessary bias current, the internal shunt
regulator can be used in conjunction with an external BJT
transistor to increase the shunt regulator current. Figure 21
details such an implementation utilizing a PNP transistor.
Selection of R1 can be based on the graphs provided for the
passive regulator configuration. Maximum power dissipation
inside Q1 will take place when maximum voltage is applied
to the circuit and the ISL6567 is disabled; determine
I_VREGMAX by reverse-use of the graph in Figure 18 and use
the obtained number to calculate Q1 power dissipation.
ISL6567
EXTERNAL CIRCUIT
PVCC
POR
CIRCUIT
E/A _-
+
VCC
V_IN
R1
Q1
R2
(optional)
VREF
VREG
SHUNT REGULATOR
FIGURE 21. INTERNAL SHUNT REGULATOR USE WITH
EXTERNAL PNP TRANSISTOR (ACTIVE
CONFIGURATION)
An NPN transistor can also be used to increase the
maximum available bias current, as shown in Figure 22.
Used as a series pass element, Q1 will dissipate the most
power when the circuit is enabled and operational, and the
input voltage, V_IN, is at its highest level.
With the series pass element configuration shown in
Figure 22, the difference between the input and the
regulation level at the VCC pin has to be higher than the
lowest acceptable V_CE of Q1 (may choose to run Q1 into
saturation, but must consider the reduced gain). Thus, R2
has to be chosen such that it will provide appropriate base
current at lowest V_CE of Q1. Next, ensure the ISL6567’s
I_VREGMAX is not exceeded when the input voltage swings to
its highest extreme (assume base current goes to 0 when
the IC is disabled). R1 is an optional circuit element: it can
be added to offset some of the power dissipation in Q1, but it
also reduces the available V_CE for Q1. If utilizing such a
series resistor, check that it does not impede on the proper
18
FN9243.2
March 20, 2007