ISL1801 Datasheet, PDF (17/30 Pages) Intersil Corporation – sPMIC for Micro Converter Bias and Drivers

ISL1801
Detailed Operation
Dual Synchronous Buck Switching
Regulators With Constant On Time Control
There are two synchronous Buck switching regulators in the
ISL1801. The high voltage switching regulator, VR1, can be
connected to a power source up to 90V. The low voltage
switching regulator, VR2, supports input voltages up to 14V.
Typically, VR2 is connected to the output of VR1. Both switching
regulators include integrated MOSFETs.
Both VR1 and VR2 employ a constant on time PWM control
architecture with input voltage feed-forward. The constant on
time PWM control architecture relies on the output ripple voltage
to provide the PWM ramp signal; thus the output filter capacitor's
ESR acts as a current feedback resistor. For some applications
with ceramic capacitors, the output voltage ripple is small due to
very low ESR. In order to achieve the stable operation for very low
voltage ripple applications, one internal ramp is generated and
added to the FB signal to emulate the output voltage ripple. The
high-side switch ON time is determined by a one-shot, which
period is inversely proportional to input voltage and directly
proportional to output voltage. Another one-shot sets a minimum
OFF time (300ns typical for VR1 and 150ns typical for VR2). The
ON time one-shot triggers when the following conditions are met;
the error comparator's output is high, the synchronous rectifier
current is below the current limit threshold, and the minimum
OFF time one-shot has timed out. The controller utilizes the valley
point of the output ripple to regulate and determine the OFF
time.
SWITCHING FREQUENCY OF VR1 AND VR2
Each PWM core includes a one-shot that sets the ON time for the
high-side switch of each voltage regulator. Each fast, low jitter,
adjustable one-shot includes circuitry that varies the ON time in
response to the input voltage and output voltage. This algorithm
results in a nearly constant switching frequency despite the lack
of a fixed-frequency clock generator.
The high-side switch ON time is inversely proportional to the
input voltage as measured by the TON1 and TON2 pins for VR1
and VR2 respectively. Both TON1 and TON2 pins are tied to an
internal voltage reference and the current flowing into these pins
is monitored to generate the ON time one-shot.
For high voltage VR1, the TON1 pin is tied to an internal 1V
reference and the width of the ON time one-shot is:
T_ON1 = ²----^.--⁷---^e-_V---^–-_I--^1N---¹-₁---^--–-^R---₁--^O----^N----¹--
(EQ. 1)
V_IN1 is the input voltage for high voltage VR1, while R_ON1 is the
resistor from V_IN1 to the TON1 pin.
The switching frequency of VR1 is:
F_SW1 = ₂----_.-^V-₇---^O_e----_–^U---₁-^T--₁--¹--_---^_R---^--_O^V----_NI--^N--₁--¹---_--–-_V---¹-_I--_N-^---₁-
(EQ. 2)
When V_IN1 is much larger than 1V, the switching frequency of
VR1 is almost independent of its input voltage:
F_SW1  -₂---_.--₇---_e---^V-_–---_1O---₁-^U---_--^T-_R--¹---_O----_N----₁--
(EQ. 3)
For a 10V output, the switching frequency of VR1 is about
370kHz with R_ON1 = 1MΩ.
For low voltage VR2, the TON2 pin is tied to an internal 0.5V
reference and the width of the ON time one-shot is:
T_ON1 = -¹---^.--⁰---^5V----^e-_I-_N-^–---¹-₂--¹--–--^--₀-^R---_.-₅-^O----^N-----²-
(EQ. 4)
V_IN2 is the input voltage for VR2, while R_ON2 is the resistor from
V_IN2 to the TON2 pin.
The switching frequency of VR2 is:
F_SW2 = ₁----^V.--₀--^O-₅---^U-_e---_–-^T--₁-²--₁---^-_--^-_R-^V----_OI--^N--_N--²---₂--–--_--⁰-_V---^.-_I⁵--_N--^--₂--
(EQ. 5)
When V_IN2 is much larger than 0.5V, the switching frequency of
VR2 is almost independent of its input voltage:
F_SW2  -₁---_.--₀---₅----_eV---_–--^O-₁---_1U----^T_---_R-²----_O----_N-----₂-
(EQ. 6)
For a 3.3V output, the switching frequency of VR2 is about
314kHz with R_ON2 = 1MΩ.
CURRENT LIMITING OF VR1 AND VR2
To prevent the output current from becoming too high, a new ON
time pulse can start only when the current through the
synchronous MOSFET is below the current limiting threshold. This
limits the valley of the output inductor current to a fixed value,
typically 140mA for VR1 and 200mA for VR2.
The maximum peak current through the output inductor is the
sum of the current limiting threshold and the current ripple
determined by the ON time, inductor value and the input/output
voltage.
DIODE EMULATION OPERATION
To improve the efficiency for light loads, the synchronous
MOSFET is turned off when its current drops to 0. This prevents
negative current through the output inductor emulating diode
operation.
With diode emulation operation under light load conditions, the
output voltage may drop slowly after the synchronous MOSFET
turns OFF. It may take a long time for the output voltage to drop
below the reference voltage to start a new switching cycle. This
will have the effect of reducing the switching frequency under
light load conditions.
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FN8259.1
July 24, 2014