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MAX17409

1-Phase Quick-PWM GPU Controller

Maxim Integrated 

1-Phase Quick-PWM GPU Controller

Detailed Description

Free-Running, Constant On-Time PWM

Controller with Input Feed-Forward

The Quick-PWM control architecture is a pseudo-fixed-

frequency, constant-on-time, current-mode regulator

with voltage feed-forward (Figure 2). This architecture

relies on the output filter capacitor’s ESR to act as the

current-sense resistor, so the output ripple voltage pro-

vides the PWM ramp signal. The control algorithm is

simple: the high-side switch on-time is determined solely

by a one-shot whose period is inversely proportional to

input voltage, and directly proportional to output volt-

age (see the On-Time One-Shot section). Another one-

shot sets a minimum off-time. The on-time one-shot

triggers when the error comparator goes low, the induc-

tor current is below the valley current-limit threshold,

and the minimum off-time one-shot times out.

+5V Bias Supply (VCC and VDD)

The Quick-PWM controller requires an external +5V

bias supply in addition to the battery. Typically, this

+5V bias supply is the notebook’s 95% efficient +5V

system supply. Keeping the bias supply external to the

IC improves efficiency and eliminates the cost associat-

ed with the +5V linear regulator that would otherwise be

needed to supply the PWM circuit and gate drivers. If

stand-alone capability is needed, the +5V bias supply

can be generated with an external linear regulator.

The +5V bias supply must provide VCC (PWM con-

troller) and VDD (gate-drive power), so the maximum

current drawn is:

( ) IBIAS = ICC + fSW QG(LOW) + QG(HIGH)

where ICC is provided in the Electrical Characteristics

table, fSW is the switching frequency, and QG(LOW) and

QG(HIGH) are the MOSFET data sheet’s total gate-

charge specification limits at VGS = 5V.

VIN and VDD can be connected together if the input

power source is a fixed +4.5V to +5.5V supply. If the

+5V bias supply is powered up prior to the battery sup-

ply, the enable signal (SHDN going from low to high)

must be delayed until the battery voltage is present to

ensure startup.

Switching Frequency (TON)

Connect a resistor (RTON) between TON and VIN to set

the switching period (tSW = 1/fSW):

tSW = 16.3pF x (RTON + 6.5kΩ)

A 96.75kΩ to 303.25kΩ corresponds to switching peri-

ods of 167ns (600kHz) to 500ns (200kHz), respectively.

High-frequency (600kHz) operation optimizes the appli-

cation for the smallest component size, trading off effi-

ciency due to higher switching losses. This might be

acceptable in ultra-portable devices where the load

currents are lower and the controller is powered from a

lower voltage supply. Low-frequency (200kHz) opera-

tion offers the best overall efficiency at the expense of

component size and board space.

On-Time One-Shot

The core contains a fast, low-jitter, adjustable one-shot

that sets the high-side MOSFET’s on-time. The one-shot

varies the on-time in response to the input and feed-

back voltages. The main high-side switch on-time is

inversely proportional to the input voltage as measured

by the RTON input, and proportional to the feedback

voltage (VFB):

t ON(MAIN)

=

t SW

(VFB + 0.075V)

VIN

where the switching period (tSW = 1/fSW) is set by the

resistor at the TON pin and 0.075V is an approximation

to accommodate the expected drop across the low-

side MOSFET switch.

This algorithm results in a nearly constant switching fre-

quency and balanced inductor currents despite the lack

of a fixed-frequency clock generator. The benefits of a

constant switching frequency are twofold: first, the fre-

quency can be selected to avoid noise-sensitive

regions such as the 455kHz IF band; second, the induc-

tor ripple-current operating point remains relatively con-

stant, resulting in easy design methodology and

predictable output-voltage ripple. The on-time one-

shots have good accuracy at the operating points

specified in the Electrical Characteristics table. On-

times at operating points far removed from the condi-

tions specified in the Electrical Characteristics table

can vary over a wider range.

On-times translate only roughly to switching frequen-

cies. The on-times guaranteed in the Electrical

Characteristics table are influenced by switching

delays in the external high-side MOSFET. Resistive

losses, including the inductor, both MOSFETs, output

capacitor ESR, and PCB copper losses in the output

and ground tend to raise the switching frequency at

higher output currents. Also, the dead-time effect

increases the effective on-time, reducing the switching

frequency. It occurs only during forced-PWM operation

and dynamic output-voltage transitions when the induc-

tor current reverses at light or negative load currents.

With reversed inductor current, the inductor’s EMF

causes LX to go high earlier than normal, extending the

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