1.5A, 280kHz, Boost Regulator
LM5171
junction temperature exceeds 180°C±30°. However, repeated operation at such high temperatures will ensure a
reduced operating life.
Calculation of the junction temperature is an imprecise but simple task. First, the power losses must be
quantified. There are three major sources of power loss on the LM5171:
• biasing of internal control circuitry, PBIAS
• switch driver, PDRIVER
• switch saturation, PSAT
The internal control circuitry, including the oscillator and linear regulator, requires a small amount of power
even when the switch is turned off. The specifications section of this datasheet reveals that the typical operating
current, IQ, due to this circuitry is 5.5mA. Additional guidance can be found in the graph of operating current vs.
temperature. This graph shows that IQ is strongly dependent on input voltage, VIN, and temperature. Then
PBIAS = VIN IQ
Since the onboard switch is an NPN transistor, the base drive current must be factored in as well. This
current is drawn from the VIN pin, in addition to the control circuitry current. The base drive current is listed in the
specifications as ΔICC/ΔISW, or switch transconductance. As before, the designer will find additional guidance in
the graphs. With that information, the designer can calculate
ICC
PDRIVER = VIN ISW
×
× D
ΔISW
where:
ISW = the current through the switch;
D = the duty cycle or percentage of switch on−time.
ISW and D are dependent on the type of converter. In a boost converter,
1
ISW(AVG) ≈ILOAD × D×
Efficiency
VOUT -VIN
VOUT
D ≈
In a flyback converter,
VOUT IILOAD
1
ISW(AVG)
≈
×
VIN
Efficiency
VOUT
NS
D ≈
VOUT
+
VIN
NP
The switch saturation voltage, V(CE)SAT, is the last major source of on−chip power loss. V(CE)SAT is the
collector−emitter voltage of the internal NPN transistor when it is driven into saturation by its base drive current.
The value for V(CE)SAT can be obtained from the specifications or from the graphs, as “Switch Saturation Voltage.”
Thus,
PSAT = V(CE)SAT ISW * D
Dec. 2010 - Rev. 1.2.1
- 18 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
