ISL6236 Datasheet, PDF (30/35 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236
VEN LDO
Low
“>2.5” → High
“>2.5” → High
“>2.5” → High
“>2.5” → High
“>2.5” → High
“>2.5” → High
VEN1 (V)
Low
Low
High
High
Low
High
REF
TABLE 4. SHUTDOWN AND STANDBY CONTROL LOGIS
VEN2 (V)
LDO
SMPS1
Low
Off
Off
Low
On
Off
High
On
On
Low
On
On
High
On
Off
REF
On
On
High
On
On (after SMPS2 is up)
SMPS2
Off
Off
On
Off
On
On (after SMPS1 is up)
On
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.
R₁
=
R₂
⋅
⎛
⎜
⎝
V_{-----O----U----T----1-}
V_FB1
–
⎞
1⎟
⎠
(EQ. 6)
where V_FB1 = 0.7V nominal.
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 0V to 2.50V for SMPS2 tracking mode
(Figure 77).
R₃
=
R₄
⋅
⎛
⎝
------^V----^R--------
V_OUT2
–
1_⎠⎞
(EQ. 7)
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_IN(MAX))
must accommodate the maximum AC adapter voltage.
The minimum value (V_IN(MIN)) must account for the
lowest input voltage after drops due to connectors, fuses
and battery selector switches. Lower input voltages result
in better efficiency.
2. Maximum Load Current. The peak load current
(I_LOAD(MAX)) determines the instantaneous component
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_LOAD) determines the thermal stress and drives
the selection of input capacitors, MOSFETs and other
critical heat-contributing components.
3. Switching Frequency. This choice determines the basic
trade-off between size and efficiency. The optimal
frequency is largely a function of maximum input voltage
and MOSFET switching losses.
4. Inductor Ripple Current Ratio (LIR). LIR is the ratio of
the 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 the controller to trigger out-of-bound
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.
V_IN
UGATE1
Q3
ISL6236
LGATE1
Q4
OUT1
OUT1
R1
FB1
R2
FIGURE 76. SETTING V_OUT1 WITH A RESISTOR DIVIDER
30
FN6373.6
April 29, 2010