US20010028240A1

US20010028240A1 - Regulator

Info

Publication number: US20010028240A1
Application number: US09/778,237
Authority: US
Inventors: Atsuo Fukui
Original assignee: Atsuo Fukui
Current assignee: Ablic Inc
Priority date: 2000-03-31
Filing date: 2001-02-07
Publication date: 2001-10-11
Anticipated expiration: 2021-02-07
Also published as: JP2001282372A; TW526405B; KR100655203B1; HK1041322B; KR20010095164A; HK1041322A1; CN1320852A; CN100403207C; US6420857B2

Abstract

To provide a regulator for restraining a variation in a frequency band and having a transient response characteristic which does not depend upon load current, by generating current in proportion to load current by a load current detecting transistor connected in parallel with an output driver transistor for supplying current to a load and changing a resistance value of a variable resistance portion by the current, a frequency of a zero point for phase compensation is varied and by varying the frequency of the zero point for phase compensation in accordance with the load current, the variation in the frequency band of the regulator is restrained without depending upon the load current and the transient response characteristic is improved.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to phase compensation for providing a transient response characteristic which does not depend on load current of a regulator.

2. Description of the Related Art

FIG. 4 shows a constitution of a conventional regulator. A reference

voltage power supply

201 supplies constant voltage Vref to an inverted input terminal of a transconductance amplifier 202. An output of the transconductance amplifier 202 is connected to the gate of a PMOS output driver transistor 204 and a phase compensating RC network 203 constituted by a resistor 208 and a capacitor 209. The source of the PMOS output driver transistor 204 is connected to an input terminal IN and the drain is connected to an output terminal OUT. The output terminal OUT is connected with a load resistor 207 and a capacitor 206 and a voltage dividing circuit 205 constituted by

resistors

210 and 211. The voltage dividing circuit 205 supplies voltage produced by dividing output voltage VOUT to a noninverting input terminal of the transconductance amplifier.

When a resistance value of the

resistor

208 constituting the phase compensation RC network 203 is designated by notation R208 and a capacitance value of the capacitor 209 is designated by notation C209, frequency fz of a zero point for phase compensation constituted by R208 and C209, is calculated by the following equation.

\begin{matrix} fz = \frac{1}{2 π \cdot R208 \cdot C209} & (1) \end{matrix}

When a resistance value of the

load resistor

207 is designated by notation R207 and a capacitance value of the load capacitor 206 is designated by notation C206, frequency fp of a pole constituted thereby is calculated by the following equation.

\begin{matrix} fp = \frac{1}{2 π \cdot R207 \cdot C206} & (2) \end{matrix}

As is apparent from Equation (2), in accordance with a variation in the

load resistor

207, the frequency fp of the pole is also changed. Meanwhile, as is apparent from Equation (1), the frequency fz of the zero point for phase compensation is a fixed value.

When load current is large, the

load resistor

207 becomes small and accordingly, by Equation (2), the frequency fp of the pole is moved to a high frequency side. Further, when the load current is small, the load resistor 207 becomes large and accordingly, by Equation (2), the frequency fp of the pole is moved to a low frequency side. FIG. 5 shows frequency characteristics of the regulator when the load current is large and when the load current is small.

As shown by FIG. 5, when the load current is large, unity gain frequency at which voltage gain of the regulator becomes 1, becomes high, conversely, when the load current is small, the unity gain frequency becomes low. When the unity gain frequency is changed by the load current in this way, the transient response characteristic depends on the load current, which is not preferable. Particularly, when the load current is small, the unity gain frequency is low and accordingly, the transient response characteristic is deteriorated.

SUMMARY OF THE INVENTION

In order to resolve the above-described problem, according to the invention, there is carried out an improvement in which by varying a frequency of a zero point for phase compensation in accordance with load current, a variation in a frequency band of a regulator is restrained such that transient response does not depend upon the load current.

According to the invention, by generating current in proportion to load current by a load current detecting transistor connected in parallel with an output driver transistor for supplying current to a load and changing a resistance value of a variable resistance portion by the current, a frequency of a zero point for phase compensation is varied.

An improvement is carried out by varying the frequency of the zero point for phase compensation in accordance with the load current, thereby, a variation in a frequency band of a regulator is restrained without depending upon the load current such that transient response does not depend upon the load current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a regulator according to a first embodiment of the invention; [0013]
FIG. 2 is a circuit diagram of a regulator according to a second embodiment of the invention; [0014]
FIG. 3 is a diagram of frequency characteristics of the regulator according to the second embodiment of the invention. [0015]
FIG. 4 is a circuit diagram of a regulator of a related art; and [0016]
FIG. 5 is a diagram of frequency characteristics of the regulator of the related art.[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An explanation will be given of embodiments of the invention in reference to the drawings as follows. [0018]
FIG. 1 shows a regulator according to a first embodiment of the invention. The reference [0019] voltage power supply 201 supplies the constant voltage Vref to the inverted input terminal of the transconductance amplifier 202. The output of the transconductance amplifier 202 is connected to the gate of the PMOS output driver transistor 204, the gate of a load current detecting PMOS transistor 212 and a phase compensation RC network 203 constituted by the capacitor 209 and a variable resistance portion 215. The source of the PMOS output driver transistor 204 is connected to the input terminal IN and the drain is connected to the output terminal OUT. The output terminal OUT is connected with the load resistor 207, the capacitor 206 and the voltage dividing circuit 205 constituted by the resistors 210 and 211. The voltage dividing circuit 205 supplies voltage produced by dividing the output voltage VOUT to the noninverted input terminal of the transconductance amplifier. The source of the load current detecting PMOS transistor 212 is connected to the input terminal IN and the drain is connected to the variable resistance portion 215.
When a gate width of the [0020] output driver transistor 204 is designated by notation W204, a gate length thereof is designated by L204, a gate width of the load current detecting transistor 212 is designated by W212 and a gate length thereof is designated by notation L212. Further, when drain current of the output driver transistor 204 is designated by notation I204 and drain current of the load current detecting transistor 212 is designated by notation I212, the following relationship is established. $\begin{matrix} I212 = \frac{W212 / L212}{W204 / L204} \cdot I204 & (3) \end{matrix}$
The drain current I[0021] 204 of the output driver transistor 204 is current supplied to load and accordingly, the drain current I212 of the load current detecting transistor 212 becomes current in proportion to the load current and the proportional coefficient is given from Equation (3) as follows. $\begin{matrix} \frac{W212 / L212}{W204 / L204} & (4) \end{matrix}$
An arbitrary proportional coefficient can be set by pertinently adjusting gate sizes of the [0022] transistors 204 and 212.
In accordance with the Equation (3), the drain current I[0023] 212 in proportion to the load current, outputted from the load current detecting transistor 212 is inputted to the variable resistance portion 215. The variable resistance portion 215 changes a resistance value thereof in accordance with inputted current.
FIG. 2 shows an embodiment further specifying the [0024] variable resistance portion 215. The variable resistance portion 215 is constituted by a resistor 213 and an NMOS transistor 214. By flowing the drain current I212 outputted from the load current detecting transistor 212 and in proportion to the load current and current I216 outputted from a constant current source 216, in the resistor 213, voltage is generated across both ends of the resistor 213. By the voltage generated across the both ends of the resistor 213, ON resistance of the NMOS transistor 214 is changed. Further, the constant current source 216 operates such that the NMOS transistor 214 is not brought into a nonconductive state even when the drain current I212 of the load current detecting transistor 212 becomes null.
As described above, ON resistance of the [0025] NMOS transistor 214 operating as phase compensation resistor is changed in accordance with the load current and accordingly, from Equation (1), the frequency fz of the zero point for phase compensation is also changed. The frequency characteristics of the regulator become as shown by FIG. 3 and even when the load current is changed, by restraining a variation in the unity gain frequency, the frequency characteristic of the regulator is improved such that transient response does not depend upon the load current.
According to the invention, by generating current in proportion to the load current by the load current detecting transistor connected in parallel with the output driver transistor for supplying current to the load and changing the resistance value of the variable resistance portion by the current, the frequency of the zero point for phase compensation is varied. [0026]

Claims

What is claimed is:

1. A circuit characterized in providing a transient response characteristic which does not depend upon load current by restraining a variation in a frequency band of a regulator by load current by changing a frequency of a zero point for phase compensation by changing a resistance value of a phase compensation RC network in accordance with the load current in the regulator.

2. A regulator comprising:

a load current detecting transistor connected in parallel with an output driver transistor for supplying current to a load;

a phase compensation RC network connected to an output end of the output driver transistor; and

a variable resistor of the phase compensation RC network connected to an output end of the load current detecting transistor.