isl6236 Intersil Corporation, isl6236 Datasheet - Page 31

• Manufacturer Part Number: isl6236
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

Adjustable-Output Feedback (Dual-Mode FB)
Connect FB1 to GND to enable the fixed 5V or tie FB1 to
VCC to set the fixed 1.5V output. Connect a resistive
voltage-divider at FB1 between OUT1 and GND to adjust the
respective output voltage between 0.7V and 5.5V
(Figure 76). Choose R2 to be approximately 10k and solve
for R1 using Equation 6.
where V
Likewise, connect REFIN2 to VCC to enable the fixed 3.3V
or tie REFIN2 to VREF3 to set the fixed 1.05V output. Set
REFIN2 from 0 to 2.50V for SMPS2 tracking mode
(Figure 77).
where:
• VR = 2V nominal (if tied to REF)
or
• VR = 3.3V nominal (if tied to VREF3)
Design Procedure
Establish the input voltage range and maximum load current
before choosing an inductor and its associated ripple current
ratio (LIR). The following four factors dictate the rest of the
design:
1. Input Voltage Range. The maximum value (V
2. Maximum Load Current. The peak load current
3. Switching Frequency. This choice determines the basic
4. Inductor Ripple Current Ratio (LIR). LIR is the ratio of the
R3
R1
must accommodate the maximum AC adapter voltage.
The minimum value (V
lowest input voltage after drops due to connectors, fuses
and battery selector switches. Lower input voltages result
in better efficiency.
(I
stress and filtering requirements and thus drives output
capacitor selection, inductor saturation rating and the
design of the current-limit circuit. The continuous load
current (I
the selection of input capacitors, MOSFETs and other
critical heat-contributing components.
trade-off between size and efficiency. The optimal
frequency is largely a function of maximum input voltage
and MOSFET switching losses.
peak-peak ripple current to the average inductor current.
Size and efficiency trade-offs must be considered when
setting the inductor ripple current ratio. Low inductor
values cause large ripple currents, resulting in the
smallest size, but poor efficiency and high output noise.
Also, total output ripple above 3.5% of the output
regulation will cause controller to trigger out-of-bound
=
LOAD(MAX)
=
R4
R2
FB1
⋅
⋅
⎛
⎝
⎛
⎜
⎝
= 0.7V nominal.
------------------ - 1
V
LOAD
V
------------------ - 1
V
OUT2
VR
OUT1
FB1
) determines the instantaneous component
) determines the thermal stress and drives
–
–
⎞
⎠
⎞
⎟
⎠
IN(MIN)
31
) must account for the
IN(MAX)
(EQ. 6)
(EQ. 7)
)
ISL6236
FIGURE 77. SETTING V
FIGURE 76. SETTING V
condition. The minimum practical inductor value is one
that causes the circuit to operate at critical conduction
(where the inductor current just touches zero with every
cycle at maximum load). Inductor values lower than this
grant no further size-reduction benefit.
The ISL6236 pulse-skipping algorithm (SKIP# = GND)
initiates skip mode at the critical conduction point, so the
inductor's operating point also determines the load
current at which PWM/PFM switchover occurs. The
optimum LIR point is usually found between 25% and
50% ripple current.
ISL6236
ISL88732
ISL88732
ISL88733
ISL6236
ISL88733
ISL6236
ISL88734
ISL88734
UGATE
UGATE2
UGATE
LGATE
LGATE2
LGATE
REFIN2
VOUT
VOUT
UGATE2
LGATE2
UGATE1
LGATE1
OUT2
REFIN2
FB
FB
OUT2
OUT1
FB1
TRACKING
R4
OUT2
OUT1
WITH A VOLTAGE DIVIDER FOR
Q4
V
Q3
V
Q1
WITH A RESISTOR DIVIDER
IN
Q2
Q2
Q2
Q2
IN
R3
R1
R2
VR
VR
June 28, 2007
OUT1
FN6373.3
OUT2