ISL6567_07 Datasheet, PDF (17/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
General Application Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multi-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced below. In
addition to this guide, Intersil provides complete reference
designs that include schematics, bills of materials, and
example board layouts for typical applications.
ISL6567
EXTERNAL CIRCUIT
PVCC
POR
CIRCUIT
VCC
V_IN
R_BIAS
E/A _-
+
VREF
SHUNT REG
VREG
FIGURE 17. INTERNAL SHUNT REGULATOR USE WITH
EXTERNAL RESISTOR (PASSIVE
CONFIGURATION)
BIAS SUPPLY CONSIDERATIONS
The ISL6567 features an on-board shunt regulator capable of
sinking up to 100mA (minimally). This integrated regulator can
be used to produce the necessary bias voltage for the
controller and the MOSFETs. The integrated regulator can be
utilized directly, via a properly sized resistor, as shown in
Figure 17, or via an external NPN transistor and additional
resistors when either the current needed or the power being
dissipated becomes too large to be handled inside the
ISL6567 in the given operating environment.
A first step in determining the feasibility and selecting the
proper bias regulator configuration consists in determining
the maximum bias current required by the circuit. While the
bias current required by the ISL6567 is listed in the Electrical
Specifications table, the bias current required by the
controlled MOSFETs needs be calculated. The following
equation helps determine this bias current function of the
sum of the gate charge of all the controlled MOSFETs at 5V
V_GS, Q_GTOTAL, and the switching frequency, F_SW:
I_B ≅ Q_GTOTAL ⋅ F_SW
I_BIAS = I_VCC + I_B
Total required bias current, I_BIAS, sums up the ISL6567’s
bias current, I_VCC, to that required by the MOSFETs, I_B.
90%
ΔV_IN = 1V
80% _{ΔVIN = 2V}
70%
ΔV_IN = 3V
ΔV_IN = 4V
60%
50%
40%
30%
20%
ΔV_IN = 5V
ΔV_IN = 6V
10%
0%
ΔV_IN = 7V
ΔV_IN = 8V
6
7
8
9
10
11
V_INmin (V)
FIGURE 18. NORMALIZED MAXIMUM BIAS CURRENT
OBTAINABLE IN PASSIVE CONFIGURATION vs
INPUT VOLTAGE RANGE CHARACTERISTIC;
V_VCC = 5V
The maximum bias current, I_BIAS, that can be obtained via
the internal shunt regulator and a simple external resistor is
characterized in Figure 18 and can also be determined using
the following equation:
I_BIASMAX
=
I_V
R
EG
M
A
X
⋅
-^V-----^I--^N----^M----^I--^N------–----^V----^V----^C-----^C--
V_INMAX – V_VCC
To exemplify the use, for an input voltage ranging from 10V
to 14V, find the intersection of the ΔV_IN = 4V curve with the
V_INmin = 10V mark and project the result on the Y axis to
find the maximum bias current obtainable (approximately
56% of the maximum current obtainable via the integrated
shunt regulator, I_{VREG_MAX}).
Once the maximum obtainable bias current, I_{BIAS_MAX}, is
determined, and providing it is greater than the bias current,
I_BIAS, required by the circuit, R_SHUNT can be determined
based on the lowest input voltage, V_INMIN:
R_BIAS
=
V_{-----I--N----M-----I-N------–----}V_{----V----C-----C--}
I_BIAS
Figure 19 helps with a quick resistor selection based on the
previous guidelines presented. Divide the value thus
obtained by the maximum desired bias current, I_BIAS, to
obtain the actual resistor value to be used.
17
FN9243.2
March 20, 2007