Internal ramp compensation for constant on-time buck converter
An integrated circuit device includes: one or more switches of a Buck converter; and a control circuit for the Buck converter including: a comparator configured to generate a comparator output signal by comparing a feedback voltage of the Buck converter with a compensation ramp voltage; a pulse generator configured to generate, in response to a rising edge in the comparator output signal, a pulse signal for controlling the Buck converter; a transconductance amplifier configured to generate, at an output terminal of the transconductance amplifier, a current proportional to a difference between a reference voltage and the feedback voltage; a capacitor coupled between the output terminal of the transconductance amplifier and a reference voltage node; and a ramp signal generator configured to generate the compensation ramp voltage by adding a voltage at the output terminal of the transconductance amplifier and a ramp voltage generated by the ramp signal generator.
1 . A switched-mode power supply (SMPS) comprising:
a Buck converter; and
a control circuit for the Buck converter, comprising:
a comparator configured to compare a feedback voltage of the Buck converter with a compensation ramp voltage, wherein a non-inverting input terminal of the comparator is configured to be coupled to the compensation ramp voltage, and an inverting input terminal of the comparator is configured to be coupled to the feedback voltage;
a pulse generator coupled to an output terminal of the comparator and configured to generate a pulse signal in response to a rising edge in an output signal of the comparator;
a logic circuit coupled to an output terminal of the pulse generator and configured to generate, based on the pulse signal, a switching control signal that controls switching of the Buck converter;
a transconductance amplifier, wherein a first input terminal of the transconductance amplifier is configured to receive the feedback voltage, and a second input terminal of the transconductance amplifier is configured to receive a reference voltage;
a first capacitor coupled between an output terminal of the transconductance amplifier and a reference voltage node; and
a ramp signal generator configured to generate the compensation ramp voltage by adding a first voltage at the output terminal of the transconductance amplifier and a ramp voltage generated by a ramp circuit of the ramp signal generator, wherein the ramp voltage is a non-linear voltage signal, wherein in a switching period of the Buck converter, a gradient of the non-linear voltage signal decreases continuously from a beginning of the non-linear voltage signal to an end of the non-linear voltage signal.
2 . The SMPS of claim 1 , further comprising an edge detector coupled between the logic circuit and the ramp signal generator, wherein the edge detector is configured to generate a control signal for the ramp signal generator by detecting a rising edge in the switching control signal.
3 . The SMPS of claim 1 , wherein at a time instant t x within the switching period of the Buck converter, the non-linear voltage signal has a gradient S x , wherein a product between t x and S x remains a same over a plurality of time instants within a duration of the non-linear voltage signal.
4 . The SMPS of claim 1 , wherein pulses in the pulse signal have a fixed pulse width.
5 . The SMPS of claim 1 , wherein at a time instant t x within the switching period, a corresponding gradient of the non-linear voltage signal at the time instant t x is equal to or larger than a gradient S x , where
S
x
=
K
×
Δ
V
o
u
t
t
x
,
ΔV out is the Buck converter's output voltage ripple, and K is a constant value.
6 . The SMPS of claim 1 , wherein the switching control signal comprises a high-side switching signal and a low-side switching signal for controlling switching of a high-side switch of the Buck converter and a low-side switch of the Buck converter, respectively.
7 . The SMPS of claim 1 , wherein the ramp signal generator comprises:
a current source configured to provide a supply current;
an operational amplifier, wherein a non-inverting input terminal of the operational amplifier is coupled to the output terminal of the transconductance amplifier, and an inverting input terminal of the operational amplifier is coupled to an output terminal of the operational amplifier;
the ramp circuit coupled between an output terminal of the current source and the output terminal of the operational amplifier, wherein a voltage at the output terminal of the current source is the compensation ramp voltage; and
a switch coupled in parallel with the ramp circuit, wherein the switch is configured to be closed for a period of time at a beginning of the switching period of the Buck converter.
8 . The SMPS of claim 7 , wherein the supply current has a constant current value, wherein the ramp circuit comprises:
a resistor coupled between the output terminal of the current source and the output terminal of the operational amplifier; and
a second capacitor coupled in parallel with the resistor.
9 . The SMPS of claim 7 , wherein the supply current has a current value that decreases linearly over time, wherein the ramp circuit comprises a capacitor coupled between the output terminal of the current source and the output terminal of the operational amplifier.
10 . The SMPS of claim 7 , wherein the supply current has a current value that decreases over time, and a time plot of the current value of the supply current has a hyperbola shape, wherein the ramp circuit comprises a capacitor coupled between the output terminal of the current source and the output terminal of the operational amplifier.
11 . An integrated circuit (IC) device comprising:
one or more switches of a Buck converter; and
a control circuit for the Buck converter, comprising:
a comparator configured to generate a comparator output signal by comparing a feedback voltage of the Buck converter with a compensation ramp voltage, wherein an inverting input terminal of the comparator is configured to be coupled to the feedback voltage, and a non-inverting input terminal of the comparator is configured to be coupled to the compensation ramp voltage;
a pulse generator configured to generate a pulse signal in response to a rising edge in the comparator output signal, wherein the Buck converter is configured to be controlled by a switching control signal derived from the pulse signal;
a transconductance amplifier configured to generate, at an output terminal of the transconductance amplifier, a current proportional to a difference between a reference voltage and the feedback voltage;
a first capacitor coupled between the output terminal of the transconductance amplifier and a reference voltage node; and
a ramp signal generator configured to generate the compensation ramp voltage by adding a first voltage at the output terminal of the transconductance amplifier and a ramp voltage generated by a ramp circuit of the ramp signal generator, wherein the ramp voltage is a non-linear voltage signal, wherein in a switching period of the Buck converter, a gradient of the non-linear voltage signal decreases continuously from a start of the switching period to an end of the switching period.
12 . The IC device of claim 11 , wherein the pulse signal has a fixed pulse width.
13 . The IC device of claim 11 , wherein at a time instant t x within the switching period, a corresponding gradient of the non-linear voltage signal at the time instant t x is equal to or larger than a gradient S x , where
S
x
=
K
×
Δ
V
o
u
t
t
x
,
ΔV out is the Buck converter's output voltage ripple, and K is a constant value.
14 . The IC device of claim 13 , wherein the pulse generator is coupled to a pin of the IC device, wherein the pin is configured to be coupled to a resistor having a resistance
R
=
K
RON
×
V
OUT
f
SM
,
where V OUT is a target output voltage of the Buck converter, f SW is a target switching frequency of the Buck converter, and K RON is a constant.
15 . A method of operating a Buck converter, the method comprising:
dividing an output voltage of the Buck converter to generate a feedback voltage;
sending the feedback voltage to an inverting input terminal of a comparator;
sending the feedback voltage and a reference voltage to a first input terminal of a transconductance amplifier and a second input terminal of the transconductance amplifier, respectively;
generating a compensation ramp voltage using a ramp signal generator, comprising:
generating a ramp voltage using a ramp circuit of the ramp signal generator, wherein the ramp voltage is generated to be a non-linear voltage signal, wherein in a switching period of the Buck converter, a gradient of the non-linear voltage signal decreases continuously from a start of the switching period to an end of the switching period; and
adding the ramp voltage to a first voltage at an output terminal of the transconductance amplifier;
sending the compensation ramp voltage to a non-inverting input terminal of the comparator;
generating, using a pulse generator, a pulse signal comprising pulses that correspond to rising edges in an output signal of the comparator; and
switching the Buck converter using a switching control signal derived from the pulse signal.
16 . The method of claim 15 , wherein the pulse signal is generated to have a fixed pulse width.