1.5A, 280kHz, Boost Regulator
LM5171
major sources, namely capacitor ESR and the charging/discharging of the output capacitor. In boost circuits,
when the power switch turns off, IL flows into the output capacitor causing an instant ΔV = IIN × ESR. At the same time,
current IL − IOUT charges the capacitor and increases the output voltage gradually.
VOUT Ripple
IL
Figure 27. Typical Output Voltage Ripple
(IIN - IOUT )(1- D)
(COUT )(f)
IOUT D
VOUT(RIPPLE)=
+
+ IIN × ESR
(COUT )(f)
The equation can be expressed more conveniently in terms of VCC, VOUT and IOUT for design purposes as follows :
IOUT (VOUT -V CC
(COUT )(f)
)
(IOUT )(VOUT )(ESR)
V CC
1
VOUT(RIPPLE)
The capacitor RMS ripple current is :
IRIPPLE = (IIN - IOUT )2(1- D)+(IOUT )2(D) = IOUT
=
×
+
(COUT )(f)
VOUT -VCC
VCC
Although the above equations apply only for boost circuits, similar equations can be derived for flyback circuits.
Reducing the Current Limit
In some applications, the designer may prefer a lower limit on the switch current than 1.5A. An external shunt
can be connected between the VC pin and ground to reduce its clamp voltage. Consequently, the current limit of
the internal power transistor current is reduced from its nominal value.
The voltage on the VC pin can be evaluated with the equation
VC = ISW RE AV
where:
RE=63mꢀ, the value of the internal emitter resistor;
AV=5V/V, the gain of the current sense amplifier.
Since RE and AV cannot be changed by the end user, the only available method for limiting switch current below
1.5A is to clamp the VC pin at a lower voltage. If the maximum switch or inductor current is substituted into the
equation above, the desired clamp voltage will result.
A simple diode clamp, as shown in Figure 28 clamps the VC voltage to a diode drop above the voltage on
resistor R3. Unfortunately, such a simple circuit is not generally acceptable if VIN is loosely regulated.
Dec. 2010 - Rev. 1.2.1
- 14 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
