ZL8801 datasheet(21/87 Pages) RENESAS

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ZL8801

FN8614 Rev.3.00

Page 21 of 87

March 27, 2015

group devices together. If the DDC pins of the devices are not

connected together and the user depends on TON_DELAY and

TOFF_DELAY values alone to ensure device sequencing, timing

accuracy will suffer. This is due to the 0ms to 4ms delay

variability between ZL8801 devices.

Fault Spreading

Digital-DC devices can be configured to broadcast a fault event

over the DDC bus to the other devices in the group. When a fault

occurs and the device is configured to shut down on a fault, the

device will shut down and broadcast the fault event over the DDC

bus. The other devices on the DDC bus will shut down together if

configured to do so, and will attempt to restart in their prescribed

order if configured to do so.

Active Current Sharing

The PWM outputs of the ZL8801 are used in parallel to create a

dual phase power rail. The device outputs will share the current

equally within a few percent, assuming all external sensing

element variations and tolerances are negligible. Current sensing

element tolerances must be taken into account, or adjusted for

using the IOUT_CAL_GAIN and IOUT_CAL_OFFSET commands in

any application.

Figure 10 shows a typical connection for a dual phase converter.

The ZL8801 will current share between phases without utilizing

output voltage droop.

Droop resistance is used in 4-, 6- and 8-phase current sharing to

add artificial resistance in the output voltage path to control the

slope of the load line curve, calibrating out the physical parasitic

mismatches due to power train components and PCB layout.

When current sharing up to 4 sets of ZL8801s (8 phases total),

the ZL8801 uses a low-bandwidth, first-order digital current

sharing technique to balance the unequal device output loading

by aligning the load lines of member devices to a reference

device.

Upon system start-up, the lowest numbered phase is defined as

the reference phase and all other phases are member phases.

The reference phase broadcasts its current over the DDC bus. The

member phases use the reference current information to trim

their reference voltages (VMEMBER) to balance the current

loading of each device in the system.

Figure 11 shows that, for load lines with identical slopes, the

member reference voltage is increased towards the reference

voltage, which closes the gap between the inductor currents.

The relation between reference and member current and voltage

is given by Equation 8:

Where R is the value of the droop resistance.

The ISHARE_CONFIG command is used to configure the device

for active current sharing. The default setting is a standalone non

current sharing, two-phase device. A current sharing rail can be

part of a system sequencing group.

A 4-, 6- or 8-phase current sharing group must have their DDC

and SYNC pins tied together in order to achieve current sensing

and accurate phase offsets between current sharing phases.

Temperature Monitoring Using XTEMP Pin

The ZL8801 supports measurement of an external device

temperature using either a thermal diode integrated in a

processor, FPGA or ASIC, or using a discrete diode-connected

2N3904 NPN transistor. Figure 12 illustrates the typical

connections required. A noise filtering capacitor, not exceeding

100pF, may be connected close to the ZL8801 XTEMP pins for

long or noisy trace runs. The external temperature sensors can

be used to provide the temperature reading for over-temperature

and under-temperature faults. The external sensors can also be

used to provide more accurate temperature compensation for

inductor DCR current sensing by being placed close to the

inductor. When routing the XTEMP signals between the inductor

and the ZL8801, these PCB traces should be kept away from the

switch node; (node connected the inductor to the MOSFET

switches).

FIGURE 10. DUAL PHASE SIMPLIFIED CIRCUIT

VOUT

ZL8801

VIN

-R

VREFERENCE

VMEMBER

IMEMBER

IREFERENCE

IOUT

FIGURE 11. ACTIVE CURRENT SHARING



MEMBER

REFERENCE

OUT

MEMBER









(EQ. 8)

ZL8801 Datasheet(PDF) 21 Page - Renesas Technology Corp

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