ISL6236 Datasheet, PDF (27/35 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236
plus VC8. As LGATE1 transitions low on the next cycle, C₁₂
will charge C₁₄ to its voltage minus a diode drop through
D2a. Finally, C₁₄ charges C₁₅ through D2b when LAGET1
switched to high. CP output voltage is:
CP = V_OUT1 + 2 ⋅ V_LGATE1 – 4 ⋅ V_D
(EQ. 4)
where:
• V_LGATE1 is the peak voltage of the LGATE1 driver
• V_D is the forward diode dropped across the Schottkys
SECFB is used to monitor the charge pump through resistive
divider. In an event when SECFB dropped below 2V, the
detection circuit force the highside MOSFET (SMPS1) off
and the low-side MOSFET (SMPS1) on for 300ns to allow
CP to recharge and SECFB rise above 2V. In the event of an
overload on CP where SECFB can not reach more than 2V,
the monitor will be deactivated. Special care should be taken
to ensure enough normal voltage ripple on each cycle as to
prevent CP shut-down. The SECFB pin has ~17mV of
hysteresis, so the ripple should be enough to bring the
SECFB voltage above the threshold by ~3x the hysteresis,
or (2V + 3*17mV) = 2.051V. Reducing the CP decoupling
capacitor and placing a small ceramic capacitor (10pF to
47pF) in parallel with the upper leg of the SECFB resistor
feedback network (R₁ of Figure 68), will also increase the
robustness of the charge pump.
Current-Limit Circuit (ILIM) with r_DS(ON)
Temperature Compensation
The current-limit circuit employs a "valley" current-sensing
algorithm. The ISL6236 uses the ON-resistance of the
synchronous rectifier as a current-sensing element. If the
magnitude of the current-sense signal at PHASE is above
the current-limit threshold, the PWM is not allowed to initiate
a new cycle. The actual peak current is greater than the
current-limit threshold by an amount equal to the inductor
ripple current. Therefore, the exact current-limit
characteristic and maximum load capability are a function of
the current-limit threshold, inductor value and input and
output voltage.
I _PEAK
I _LOAD
ΔI
I _LOAD(MAX)
ILIM _{(VAL) = ILOAD -}
ΔI
2
I _LIMIT
TIME
FIGURE 73. “VALLEY” CURRENT LIMIT THRESHOLD POINT
For lower power dissipation, the ISL6236 uses the
ON-resistance of the synchronous rectifier as the
current-sense element. Use the worst-case maximum value
for r_DS(ON) from the MOSFET data sheet. Add some margin
for the rise in r_DS(ON) with temperature. A good general rule
is to allow 0.5% additional resistance for each °C of
temperature rise. The ISL6236 controller has a built-in 5µA
current source, as shown in Figure 74. Place the hottest
power MOSEFTs as close to the IC as possible for best
thermal coupling. The current limit varies with the
ON-resistance of the synchronous rectifier. When combined
with the undervoltage-protection circuit, this current-limit
method is effective in almost every circumstance.
ILIM
RILIM VILIM
5µA
VCC
9R ^{TO CURRENT}
LIMIT LOGIC
R
FIGURE 74. CURRENT LIMIT BLOCK DIAGRAM
A negative current limit prevents excessive reverse inductor
currents when VOUT sinks current. The negative
current-limit threshold is set to approximately 120% of the
positive current limit and therefore tracks the positive current
limit when ILIM is adjusted. The current-limit threshold is
adjusted with an external resistor for ISL6236 at ILIM. The
current-limit threshold adjustment range is from 20mV to
200mV. In the adjustable mode, the current-limit threshold
voltage is 1/10th the voltage at ILIM. The voltage at ILIM pin
is the product of 5µA*R_ILIM. The threshold defaults to
100mV when ILIM is connected to VCC. The logic threshold
for switch-over to the 100mV default value is approximately
VCC -1V.
The PC board layout guidelines should be carefully
observed to ensure that noise and DC errors do not corrupt
the current-sense signals at PHASE.
MOSFET Gate Drivers (UGATE, LGATE)
The UGATE and LGATE gate drivers sink 2.0A and 3.3A
respectively of gate drive, ensuring robust gate drive for
high-current applications. The UGATE floating high-side
MOSFET drivers are powered by diode-capacitor charge
pumps at BOOT. The LGATE synchronous-rectifier drivers
are powered by PVCC.
27
FN6373.6
April 29, 2010