PE99151DIE Datasheet, PDF (8/15 Pages) Peregrine Semiconductor – Hi-Rel 2A DC-DC Converter

PE99151 DIE
Product Specification
Theory of Operation
General
The PE99151 is a radiation-hardened point-of-load buck
regulator. This highly integrated switching regulator
contains two synchronous power switches capable of
delivering up to 2A of continuous current. The PE99151
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 PE99151 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 across all
application conditions. The PE99151 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 PE99151 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 PE99151 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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