PE99153 DIE
Product Specification
Efficiency Estimation and Improvement
High Side Switch Loss
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.
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:
IRMS–HSS = ILOAD
– ∆IL / 2 + ∆IL / √3
PLOSS–HSS = IRMS–HSS2 × RON–HSS × D
Where the extra factor of D = VOUT/VIN 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:
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.
IRMS–LSS = ILOAD
– ∆IL / 2 + ∆IL / √3
PLOSS–LSS = IRMS–LSS2 × RON–LSS × (1 –D)
Where the extra factor of 1 – (D = VOUT / VIN) is the duty
ratio of the LSS and is included because power is only
dissipated in the LSS when it is on.
The following sections give the mathematical expressions
of six main loss terms calculated in the design guide
spreadsheet.
Other Internal Loss
Input Capacitor
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.
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:
IRMS–CIN = ILOAD (max) × √[D × (1–D)]
Power lost in the input capacitor can be calculated as:
PLOSS–CIN = I2RMS–CIN × RCIN–ESR
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 8 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 VIN. 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.
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:
PLOSS–COUT = (∆
IL2/ 3) × RCOUT–ESR
Note that RCOUT–ESR is the ESR of the frequency range of
capacitors absorbing the ripple current.
Inductor
The inductor RMS current is given by:
Figure 8. PE99153 DIE Current Limits
Absolute Max: Io
ILRMS = I LOAD
– ∆IL/2 + ∆IL / √3
Operating Max: Imax
Power lost in the DC resistance of the inductor is then
given as:
Max Current Limit:
ILIMXEXT or ILIMXINT
Current Threshold
PLOSS–LOUT–DCR = ILRMS2 × RLOUT–DCR
IL
ILOAD (average current)
Time
©2012–2015 Peregrine Semiconductor Corp. All rights reserved.
Document No. DOC-50371-6 │ UltraCMOS® Power Management Solutions
Page 12 of 15
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