PE99151 DIE
Product Specification
In external RSET mode, the PE99151 senses the voltage
applied to the ISET pin (from the EAOUT pin, through the
compensation network) and subtracts the resulting
Given the required compensation current (Ma), the voltage
ramp rate at the ICOMP pin can be calculated by dividing
by the GICOMP parameter in Table 2.
reference current offset. This subtraction is then applied to
the RSET pin. This voltage at the RSET pin draws current
through the RSET resistor. This current is used as a
current threshold to set the peak inductor current in the
current mode control loop. The maximum voltage at the
RSET pin is limited to VMAXRSET in Table 2 independent
of the voltage applied to the ISET pin. Since this voltage is
limited, the maximum reference current through the RSET
∆
VICOMP = ∆ICOMP / GICOMP, where
∆ICOMP = (∆IL × ∆ton × Ma / M2) / ∆toff and ∆VICOMP =
(0.95 × VOUT2) / CICOMP × RCOMP × FSW × VIN)
Next, substituting for ∆IL from the output inductor equation
and given the required voltage ramp rate and the internal
capacitance connected to the ICOMP pin, CICOMP in
Table 2, the resistor connected from VOUT to the ICOMP pin
can be calculated:
resistor is limited to VMAXRSET / RSET
.
After a reference current is generated through the RSET
pin, the current is multiplied by the GIREF parameter listed
in Table 2. Thus the peak current allowed by the current
mode control loop will be limited to:
RCOMP = (0.95 × GICOMP × L) / (CICOMP × Ma / M2),
where 95% of VOUT is used to achieve a linear
approximation of the average current through CICOMP,
assuming the voltage drop over 1 cycle varies from 100 to
90%.
ILIMIT = GIREF × (VMAXRSET/RSET) – ∆ICOMP
Solving for RSET
Figure 9. PE99151 Slope Compensation
RSET = (GIREF × VMAXRSET) / (ILIMIT + ∆ICOMP)
Slope Compensation
Slope Compensation Ramp Selection
VMAXRSET
GIREF
ΔICOMP
=
While providing improved bandwidth and inherent current
limiting, all current mode control switching regulators
require slope compensation to ensure stability across all
applications conditions. The PE99151 provides adjustable
slope compensation to allow the designer to optimize
transient response and stability requirements.
RSET
Ma
M1
ΔIL
M2
ILOAD AVG
Δton Δtoff
Inductor Current
During steady state, a compensation ramp is created at
the ICOMP pin. The RC ramp is created by the external
RCOMP resistor and an internal capacitor. The ramp is reset
any time the low side switch is on by means of a reset
switch. The voltage at the ICOMP pin is multiplied by the
GIref parameter in Table 2 and is subtracted from the
reference current of the current threshold comparator.
Time
Voltage Control Loop Compensation
Network Design
Before calculating the required slope compensation to
ensure stability, a related slope is defined. In steady state,
when the low side switch is on, the inductor current ramps
down at a rate of M2. M2 is given as VOUT / L, where L is
the inductance of the output inductor.
The PE99151 contains a current mode control loop and a
voltage mode control loop as shown in Figure 10.
The current mode control loop, to first order, controls
average inductor current and so behaves as a current
source. Conceptually, the current mode control loop can be
replaced with a voltage controlled current source.
The minimum slope compensation ramp current, Ma
required for mathematical steady state stability under all
conditions is one half of M2. That is:
External to the compensation network, the resulting
network contains one pole in the voltage control loop. This
pole is created by the parallel combination of
Ma≥M2 / 2
Cout and the Load Resistance and is located at:
To provide margin to the minimum, a compensation slope
equal to M2 is recommended:
1 / (2πRLOADCOUT
)
Ma = M2
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