ISL6441 Datasheet, PDF (12/18 Pages) Intersil Corporation – 1.4MHz Dual, 180 Out-of-Phase, Step-Down PWM and Single Linear Controller

ISL6441
requirements for EMI. The typical operating curves show the
synchronized 180^° out-of-phase operation.
Input Voltage Range
The ISL6441 is designed to operate from input supplies
ranging from 4.5V to 24V. However, the input voltage range
can be effectively limited by the available maximum duty
cycle (D_MAX = 71%).
V_IN(min)
=
⎛
⎝
-^V----^O----^U--₀--^T--_.-₇--+--₁---^V----^d----¹-_⎠⎞
+ V_d2 – V_d1
(EQ. 3)
where,
V_d1 = Sum of the parasitic voltage drops in the inductor
discharge path, including the lower FET, inductor and PC
board.
V_d2 = Sum of the voltage drops in the charging path,
including the upper FET, inductor and PC board resistances.
The maximum input voltage and minimum output voltage is
limited by the minimum ON-time (t_ON(min)).
V_IN(max) ≤ -_t-_O-----_N----₍--_m-----_i-^V-_n---₎^O---_×-^U----₁-^T--_.--₄----_M-----_H-----_z-
(EQ. 4)
where, t_ON(min) = 30ns
Gate Control Logic
The gate control logic translates generated PWM signals
into gate drive signals providing amplification, level shifting
and shoot-through protection. The gate drivers have some
circuitry that helps optimize the IC’s performance over a
wide range of operational conditions. As MOSFET switching
times can vary dramatically from type-to-type and with input
voltage, the gate control logic provides adaptive dead time
by monitoring real gate waveforms of both the upper and the
lower MOSFETs. Shoot-through control logic provides a
20ns deadtime to ensure that both the upper and lower
MOSFETs will not turn on simultaneously and cause a
shoot-through condition.
Gate Drivers
The low-side gate driver is supplied from VCC_5V and
provides a peak sink/source current of 400mA. The
high-side gate driver is also capable of 400mA current.
Gate-drive voltages for the upper N-Channel MOSFET are
generated by the flying capacitor boot circuit. A boot
capacitor (CBOOT at Figure 15) connected from the BOOT
pin to the PHASE node provides power to the high side
MOSFET driver. It’s highly recommended to add a small
resistor (RBOOT1 at Figure 15, 5.1Ω typical) in series with
CBOOT and another small resistor (RBOOT2 at Figure 15,
5.1Ω typical) in series with the bootstrap diode to prevent the
overcharge of CBOOT that may cause overvoltage failure
between BOOT and PHASE pin (Figure 15). RBOOT1 also
functions as the resistor in series with the Ugate for damping
the upper gate driving and phase node oscillations, which
helps to improves the EMI performance. But this resistor will
slow down the turn-on of upper MOSFET, so RBOOT1 can’t
be too big. RBOOT2 only functions solely to prevent the
overcharge of CBOOT. While the RBOOT1 and RBOOT2
will introduce voltage drop and reduce the DC voltage on
CBOOT. So they can’t be too large to affect the DC driving
voltage of upper MOSFET.
VCC_5V
BOOT
UGATE
PHASE
VIN
DBOOT
CBOOT
RBOOT2 4.7µF
5.1Ω
RBOOT1
5.1Ω
CBOOT
ISL6441
FIGURE 15. GATE DRIVER
At start-up the low-side MOSFET turns on and forces
PHASE to ground in order to charge the BOOT capacitor to
5V. After the low-side MOSFET turns off, the high-side
MOSFET is turned on by closing an internal switch between
BOOT and UGATE. This provides the necessary
gate-to-source voltage to turn on the upper MOSFET, an
action that boosts the 5V gate drive signal above V_IN. The
current required to drive the upper MOSFET is drawn from
the internal 5V regulator.
Protection Circuits
The converter output is monitored and protected against
overload, short circuit and undervoltage conditions. A
sustained overload on the output sets the PGOOD low and
initiates hiccup mode.
Overcurrent Protection
Cycle by cycle current limiting scheme is implemented in
Equation 5. Both PWM controllers use the lower MOSFET’s
ON-resistance, r_DS(ON), to monitor the current in the
converter. The sensed voltage drop is compared with a
threshold set by a resistor connected from the OCSETx pin
to ground.
R_OCSET
=
---------⁽---⁷---⁾---⁽--^R-----^C----^S----⁾---------
(I_OC)(r_DS(ON))
(EQ. 5)
where, I_OC is the desired overcurrent protection threshold,
and R_CS is a value of the current sense resistor connected
to the ISENx pin. If the lower MOSFET current exceeds the
overcurrent threshold, a pulse skipping circuit is activated.
Figure 16 shows the inductor current, output voltage, and the
PHASE node voltage just as an overcurrent trip occurs. The
upper MOSFET will not be turned on as long as the sensed
current is higher than the threshold value. This limits the
current supplied by the DC voltage source. If an overcurrent
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FN9197.3
May 26, 2009