PE99153 Datasheet, PDF (11/15 Pages) Peregrine Semiconductor – Radiation Hardened UltraCMOS Monolithic Point-of-Load Synchronous Buck Regulator with Integrated Switches

PE99153
Product Specification
Efficiency Estimation and Improvement
The efficiency of a switch mode power supply can be
estimated by identifying and estimating all sources of loss
in the power supply system. These loss terms include
switching losses, resistive losses, losses incurred on chip
and losses associated with external passive components.
External passive losses occur primarily in the output
inductor, the output capacitor and the input capacitor.
Internal losses at high current are dominated by the high
and low side switch resistance. At low current, internal
losses are dominated by quiescent bias current and
switching related losses.
The PE99153 Design Guide provides a simple tool for
estimating loss. Losses are parameterized across input
voltage, output voltage and switching frequency to
provide accurate estimates of the performance of the part
under a variety of conditions.
The following sections give the mathematical expressions
of six main loss terms calculated in the design guide
spreadsheet.
Input Capacitor
The loss in the input capacitor can be calculated by using
the estimate of the RMS capacitor current calculated in
the input capacitor selection section. Given that:
I_RMS-CIN = I_{LOAD (max)} x √ [D x (1-D)]
Power lost in the input capacitor can be calculated as:
P_LOSS-CIN = I^2RMS-CIN x R_CIN-ESR
Output Capacitor
The RMS current through the output capacitor in steady
state was calculated in the output capacitor selection
section as ∆IL/√ 3. Power loss in the output capacitor is
then calculated as:
P_LOSS-COUT = (∆IL²/ 3) x R_COUT-ESR
Note that R_COUT-ESR is the ESR of the frequency range of
capacitors absorbing the ripple current.
Inductor
The inductor RMS current is given by:
IL_RMS = I _LOAD -∆IL/2 + ∆IL/√ 3
Power lost in the DC resistance of the inductor is then
given as:
P_{LOSS-LOUT-DCR} = I_LRMS2 x R_LOUT-DCR
High Side Switch Loss
During the time the HSS is on, it is supporting the load
current plus the inductor ripple current. RMS current
through the HSS, when it is on, is given by:
I_RMS-HSS = I_LOAD -∆IL/2 + ∆IL/√3
P_LOSS-HSS = I_RMS-HSS2 x R_ON-HSS x D
Where the extra factor of D = V_OUT/V_IN is the duty ratio
and is included because power is only dissipated in the
HSS when it is on.
Low Side Switch Loss
During the time the LSS is on, it is supporting the load
current plus the inductor ripple current. RMS current
through the LSS, when it is on, is given by:
I_RMS-LSS = I_LOAD -∆IL/2 + ∆IL/√ 3
P_LOSS-LSS = I_RMS-LSS2 x R_ON-LSS x (1 -D)
Where the extra factor of 1 – (D = V_OUT/V_IN) is the duty
ratio of the LSS and is included because power is only
dissipated in the LSS when it is on.
Other Internal Loss
A complete list of internal losses in the PE99153 regulator
is estimated and available in the PE99153 design guide
spreadsheet available online. The internal losses are
parameterized across input voltage, output voltage and
switching frequency to provide accurate estimates of the
performance under a variety of conditions.
Setting the Current Limit
When the RSEL pin is grounded, the PE99153 uses an
internal current limiting resistor that will limit the output
current to a value of ILIMXINT listed in Table 2 of the
datasheet. See Figure 10 for a visual description of the
various current limits. The part can be programmed to
use an alternate current limit by tying the RSEL pin to
V_IN. In this mode, the PE99153 can be programmed to
various output current limits through the selection of a
resistor connecting the RSET pin to ground.
Figure 10. PE99153 Current Limit
Absolute Max: Io
Operating Max: Imax
Max Current Limit:
ILIMXEXT or ILIMXINT
Current Threshold
IL
I_LOAD (average current)
Document No. DOC-29414-2 │ www.psemi.com
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