PE99155 Datasheet, PDF (7/15 Pages) Peregrine Semiconductor – Radiation Hardened UltraCMOS Monolithic Point-of-Load Synchronous Buck Regulator with Integrated Switches

PE99155
Product Specification
Theory of Operation
General
The PE99155 is a radiation-hardened point-of-load buck
regulator. This highly integrated switching regulator
contains two synchronous power switches capable of
delivering up to 10A of continuous current. The PE99155
is designed to operate from a wide 5V bus and provide
1.0V to 3.6V supply rails for analog, digital and RF
payloads. The internal oscillator can operate at 500 kHz
or 1 MHz. Optionally, the switching frequency can be
synchronized to an external reference from 100 kHz to
5 MHz. Current limiting is adjustable with an external
resistor and is achieved through peak current mode
control. An external resistor also provides adjustable
slope compensation to optimize stability and closed loop
bandwidth across output voltage and switching
frequency range. Loop compensation is externally
adjustable to meet application transient response while
still maintaining stability requirements. The output is tri-
stated when the SDb pin is low to enable hot-spare
capability.
Peak Current Mode Control Loop
The PE99155 uses a peak current mode control
architecture. At the falling edge of either the internal
oscillator or, if present, the external reference, the high
side switch turns on. The input voltage is then
connected to the load voltage through the high side
switch and the inductor for a time greater than the
minimum-on-time. Current in the inductor begins to
ramp approximately as (V_IN – V_OUT )/L. Energy is
stored in the inductor during this period. As the
inductor current rises, current through the high side
switch is sensed and compared to a current threshold.
The inductor current continues to ramp until the current
threshold is reached. At this point the high side switch
turns off and the low side switch turns on for at least
the minimum-off-time. Energy stored in the inductor
during the previous phase is discharged into the load
supply rail through the low side switch and the
inductor. Inductor current decreases at a rate of
approximately V_OUT/L. The low side switch stays on
until the next falling edge of the reference clock. In
order to prevent unintended harmonics or spurs, the
part does not exit continuous conduction mode.
Whether the current threshold was met in the previous
clock cycle or not, a minimum-off-time, followed by a
minimum-on-time immediately follows the falling edge of
the reference clock.
While providing improved bandwidth and inherent
current limiting, all current mode control switching
regulators require slope compensation to ensure stability
Document No. 70-0307-07 │ www.psemi.com
across all application conditions. The PE99155 provides
adjustable slope compensation to allow the designer to
optimize transient response and stability requirements.
The compensation ramp is provided through the ICOMP
pin. Inboard of the ICOMP pin is the CICOMP capacitor
which can be used to generate an RC compensation
ramp by tying the ICOMP pin to either V_OUT or V_IN
through an external resistor to produce the desired
ramp. See the Design Guide for selection of the
appropriate resistor value. The RC ramp is reset anytime
the low side switch is on by a FET switch.
Current Threshold and Over Current Protection
The current mode control threshold current is set by the
ISET pin which is driven by the voltage control loop from
the EAOUT pin. The PE99155 takes the voltage applied
to the ISET pin, subtracts 0.7V (typ) and applies that
voltage to the R_SET resistor. An internal R_SET resistor will
be used if the RSEL pin is grounded or an external R_SET
resistor connected to the RSET pin is used if the RSEL
pin is tied high. The current flowing through the R_SET
resistor is then used as a scaled current reference for
the inductor current threshold comparison. The scaling
ratio is defined as G_IREF in Table 2.
Over current protection is achieved by limiting the
maximum voltage applied to the internal or external R_SET
resistor to the VMAXRSET value listed in Table 2. Thus,
the current limit can be adjusted by selection of the
external R_SET resistor. This flexibility allows
characterization and testing to a high current in the lab
while still limiting the current to lower level in the
application.
Voltage Control Loop
The output voltage is achieved by controlling the ISET
pin. The PE99155 contains an amplifier with both of the
positive and negative input terminals, EAINP and EAINM
respectively, and the output terminal EAOUT all pinned
out to package pins. This allows for flexible
configurations of the voltage reference, error amplifier,
feedback networks and the current mode control loop. In
normal configuration the error amp senses the output
voltage, V_OUT, through a resistor divider that produces a
1.000V division at the target V_OUT. It compares that
feedback voltage to the 1.000V reference and increases
the voltage applied to the ISET pin when the output
voltage is low and decreases the voltage applied to the
ISET pin when the output voltage is high. Loop
compensation is required to attenuate the frequency
content at and above the switching frequency and to
achieve the desired phase margin in the voltage control
loop. See the Design Guide for instructions on designing
the compensation network.
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